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How to Achieve Proper Circulation in an Aquarium
by Pearl A.
Functions of Circulation || Natural Water Movement || GPH || Filters
Air Pumps & Air Stones || Water Pumps || Powerheads & Wavemakers
Freshwater & Marine Scenarios || Conclusions
Water circulation is an essential and often misunderstood part of successful aquarium keeping. Some of the confusion is probably linked to the variety of devices that create circulation - water pumps, powerheads, filters, even air pumps. What do you actually need for your setup? How will different combinations of equipment affect those needs?
This article is here to help you understand aquarium water flow, and the equipment that creates it. We'll also discuss different circulation needs for different types of aquariums, so you can make smart choices when planning circulation or improving your setup.
FUNCTIONS OF CIRCULATION
Consider this example: the betta fish comes from small puddles or ponds, where it has adapted to breathe the air above the water's surface. This is why we can keep them, unlike other fish, in tiny bowls without circulation or filtration. But betta bowls need most or all of their water changed frequently, more frequently than larger tanks. If these fishes live naturally in stagnancy, why is this so important?
The unique isolation of a closed aquarium is significantly different from ponds or swamps, where stagnancy is offset by a great variety of plant and microbial life, changing weather conditions, and other dynamic processes. An aquarium is not a complete ecosystem, and the task of the aquarist is to create and maintain the stability and balance that occurs in nature.
Stagnancy has profound effects on a number of critical processes in the aquarium.
Gas Exchange
Imagine beating egg whites with a wire whisk. They become stiff, foamy peaks, because you're folding air into the whites by breaking their surface tension. That's how gas exchange occurs in water. Water movement creates the surface agitation needed to bring in oxygen, while beating out waste gases like nitrogen that need to escape.
Gas exchange occurs in still waters too, through the aquatic plants which take root where there is low water movement. They consume CO2 and release oxygen during the daytime (they use oxygen at night). They also consume nitrates, which would otherwise be converted into nitrogen gas by your biological filter. Thus the water's surface isn't the primary site of gas exchange in natural still waters.
However, in a planted aquarium you have other considerations (getting CO2 into your system in the day, getting oxygen into your system at night). And you'll still need to add minimal circulation to a planted aquarium to address some of the issues below. (More on planted tanks later.)
Oxygen and other Essential Elements
The amount of oxygen naturally dissolved into your water through surface gas exchange won't sustain livestock and the bacteria that process their waste, which use oxygen as well. Nor will surface movement distribute oxygen evenly throughout your tank.
"Dead spots" or anaerobic zones form when circulation is inadequate, especially in pockets of substrate and obstructed areas where flow can't reach. Dead spots are dangerous not only because they lack oxygen, but because the anaerobic bacteria that thrive there produce harmful chemicals. The same logic applies to other essential elements, like the trace elements needed by reef invertebrates, which rely on water movement to carry nutrients to them.
Filtration
Filters can't do much to clean and detoxify your tank if your water isn't moving through them a) completely and b) frequently. Even filters with pumps slow the flow of water as it passes through membranes and media inside. Since it's slowing down not once but every time it passes through, this can have a big effect when filters start to clog, especially if they are your only means of circulation.
Water Conditions
Proper water circulation is important to stabilize water conditions like temperature and salinity, where fluctuations can stress and even kill your organisms. Aquarium heaters and chillers affect only one area within your tank, relying on circulation to move water through this area for even heating or cooling. Salinity depends on dissolved salts, which with inadequate circulation can settle or "precipitate" creating false readings and potentially dangerous imbalances.
Algae
Normal algal growth is unavoidable, but discouraged by water movement. It can't form an ugly film on your water's surface, nor can it take hold on other surfaces as easily when water is constantly moving. In a planted tank, the natural competition for nutrients from other plants generally keeps algae in check. In other tanks, circulation can make the difference.
NATURAL WATER MOVEMENT
Underwater creatures have physiologically adapted to the way water flows in their natural environments. The importance of this is often underestimated, but when you think about it, makes a great deal of sense. Fishes used to strong currents are strong swimmers and moreover, need the exercise to maintain metabolic health, easily becoming overweight and unhealthy in still waters. Fishes unaccustomed to water movement lack the strength and stamina to deal with it, becoming stressed and even dying if forced swim constantly.
Adaptation to water movement is more pronounced when it comes to invertebrates. Corals, anemones, coralline algae, mollusks - these creatures developed to rely on vigorous water movement for all of their biological needs. Food and trace elements must be pushed through them, waste carried away; even spawning requires the help of currents. Plus, moving water cleans corals of sediment deposits that form in the carbonate rich waters of the reef - deposits which arrest growth, block feeding, dull colors, and generally impede good health.
It's not simply enough to observe how much water movement is present in the natural environment of your aquarium community. The type of water movement is important, and in fact, quite helpful in determining your setup.
Laminar currents - steady, uni-directional current found in rivers
Surge - strong current in one direction followed by weaker reverse current; found in oceans, it's the movement that provokes the characteristic back and forth motion of schools of fish
Turbulence - random water movement by multi-directional currents; found in oceans in combination with surge patterns
Lakes and Ponds - no strong current flow in one or any direction; even and gradual water movement.
We'll go over specific habitats in more detail towards the end of the article (or skip ahead if you'd like). First, with our new understanding of circulation, we can start breaking it down in terms of equipment.
GPH
GPH, or gallons per hour, is the unit used to measure water movement. Once you know how big your tank will be and what your turnover rate should be, you can calculate what GPH you need from your pumps and filters. For instance, if your tank is 50 gallons, and needs to be "turned over" 4 times in an hour, you'll need a pump rated 200 GPH (50 x 4).
Turnover rates vary to some degree, but here are a few general guidelines:
Tank Type Turnover Rate
Freshwater Fish-Only 4 times per hour
Planted Tanks 2 times per hour
Fry Tanks or Tanks with Small Freshwater Fishes or Bettas 2 times per hour
Freshwater Tanks with Large Specimens (Increased Waste) 5-6 times per hour
Saltwater Fish-Only 5-10+ times per hour
Saltwater Reef 10-20+ times per hour
That said, aim a little higher than you need. Adjust the turnover rate by 1 or look for the next highest GPH rating - this will help you overcome the resistance you encounter from equipment, filter media, plumbing fittings and dirt or clogging in any of those elements.
FILTERS
Different types of filters have different flow rates. Sometimes they will list their GPH; they may just have a tank size rating which indirectly refers to their flow rate. In smaller aquariums, particularly freshwater, you can achieve good circulation with filters alone.
The drawback of using filters as primary circulation is that you'll need to be quite vigilant about regular maintenance - not just in cleaning or replacing your filter media promptly, but also in cleaning the inside of your filters and tubing every few months.
Sponge filters, undergravel filters, and box or corner filters use air pumps to push water through them at a low flow rate. They may provide sufficient circulation for small tanks where very low flow is desired, like quarantine tanks and fry tanks.
External (or HOB - hang on back) power filters and canister filters have the highest GPH ratings. Internal power filters produce less flow than their hang-on counterparts. Power filters are often the primary source of circulation in smaller tanks.
Canister filters are pressurized - they sit outside your tank, so they have to move water further. If they don't come with their own water pump, they need one to provide that extra pressure - they may be powerful enough to circulate your tank, but look for one more powerful than you need to counteract any flow loss. Wet/dry filters, also outside or under your tank, require a water pump as well (and often don't include one).
AIR PUMPS & AIR STONES
Air pumps and stones aren't designed for circulation, though they do produce a little movement. Their bubbles (or micro-bubbles, in the case of airstones), do break the water's surface tension to aid in gas exchange. But these streams of air cannot move your entire volume of water, much less evenly. This explains the lack of GPH ratings on air pumps - they tell you how much air they produce (or how large of a tank they aerate).
Therefore, an air pump/filter combo may be perfectly sufficient for small or shallow tanks, but once you get above 50 gallons, you'll need a water pump to circulate oxygen (barring a powerful pump-driven filter). Some water pumps aerate too, but it's good practice to backup aeration from filters or pumps - in case of clogs or power loss, air stones will keep going, and can save lives.
WATER PUMPS
Water pumps move water without strong currents, ideal for even circulation. Many experienced aquarists recommend 2 less powerful water pumps over 1 pump that meets your total need - this way, if one malfunctions, you have the other to buy you time to fix or replace the first. Water pumps are the best way to ensure total movement of a large tank - if you need directional flow but still want the circulating power of a normal pump, combine it with a powerhead (more in the next section).
Head Height
You will see charts in pump descriptions listing "200 GPH at 0 head height/150 GPH at 3 feet", and so on - with GPH decreasing as head height increases.
Head height is the distance from the pump to the highest point it must reach (not necessarily the surface of your tank, but wherever the output of the pump will feed to). If your head height is 0, your pump is flush with its output flow. If you'll be keeping your pump in a sump, you may have 5-6 feet of head height. All pumps will provide head height/GPH correlations, so as long as you reasonably estimate your head height you can simply refer to the GPH rating at that height.
Resistance & Plumbing
Plan on resistance from equipment and circumstances likely to slow your flow rate. A good rule is to estimate about a step up from the recommended GPH. If you're using canister filters, which are pressurized, or will need extensive plumbing, take it one step further (turn your tank 6 times instead of 4). You can always adjust your pump's GPH down, but never up, so err on the side of more power.
You will see measurements for input and output tubing in the specifications for any pump. Never use input tubing smaller than the input specifications. Some people use smaller output tubing to decrease their output flow. This strains both your tubing and your pump and results in shortened life or more immediate problems. If you want to decrease your output, attach a ball valve, which gives you greater control of adjustment anyway.
Submerged v. External
Submerged pumps are quiet and easy to install. They will produce some heat, and if you have a saltwater tank make sure any submerged equipment is saltwater safe. Often these pumps are placed in a sump, minimizing heat and clutter in your tank. In this case, you'll need to match the dimensions of your sump's pump chamber.
External pumps won't heat your tank as much, but you'll need somewhere to put them, and they can be noisier (some brands feature quiet operation, look for those if it's a concern). Usually they're configured either for high pressure use or circulation (free-flow). Pressure setups are needed if you're installing the pump in-line before your canister filter, feeding water at high pressure, or if you're operating at extreme head heights. Free flow pumps are for general circulation or returning water to your tank from your wet/dry sump.
External pumps may require modifications to your sump or filters to accommodate plumbing. However, some aquarists don't like using submerged pumps with reef systems - the calcium or kalkwasser solutions that are dosed in these systems can lead to more pump cleaning and possible lessened efficiency.
POWERHEADS & WAVEMAKERS
Powerheads are small, submersible water pumps which attach to the walls of your tank and produce laminar, or uni-directional, currents. They make a healthy addition to a river habitat, providing the right style of current for muscular river fishes to get a good workout. Powerheads are also some of the best tools for saltwater aquariums. They are inexpensive, so you can buy 2 or even more and place them at different points on your tank to replicate ocean turbulence.
There is a lot of variety among powerheads - some aerate, some have rotating returns, others can function as normal pumps. Because they are smaller and uni-directional, it's better to use 2 than 1 for circulation. You don't want to aim a powerhead straight at the popular hang-out spot in your aquarium, or directly at invertebrates - it's nice for organisms to be able to take refuge from the strong movement if they tire, and corals thrive along currents rather than in their path. (You can hand-hold a powerhead to blast particularly tough detritus from the surfaces of corals, if problem areas form.)
The other benefit of powerheads for saltwater aquariums is that some models can be attached to wavemakers. Wavemakers are basically power strips with outlets designed to alternate electrical current, switching power at intervals between powerheads to create movement close to random alternating currents. This type of vigorous water movement is ideal for reef tanks.
FRESHWATER & MARINE SCENARIOS
Here are some setup suggestions for scenarios you may encounter. With basic plumbing skills, you can design custom setups with any flow directionality you can dream up, but we'll stick to commonly used equipment here.
Small Freshwater Bowls/Tanks up to 3 gallons:
Biweekly water changes of 20% or more
No additional circulation
Small Fry Tanks, Quarantine Tanks and Tanks with Small or Pond Fishes:
Turn over 2x/ hour
For fry and quarantine tanks, an air pump-driven sponge filter provides the circulation you need without damaging fish or removing medicines.
For small fish tanks or tanks with betta or goldfish, an internal corner or box filter, also air pump-driven, can be used instead if preferred, although a sponge filter is still a good choice. Look for an air pump with multiple outlets, so you can add an air stone for improved surface gas exchange.
Freshwater Fish Only Tanks:
Turn over 4x/hour
In tanks with large specimens, higher GPH (5 or 6x/hr) assists with increased waste.
Power or canister filters alone can circulate tanks up to 50 gal or so; add an air pump & airstones to avoid oxygen shut down in case of filter clogs or power outages.
Add an aerating water pump or a water pump/air pump combo to tanks larger than 50 gallons to prevent anaerobic dead spots.
Freshwater Planted:
Turn over 2x per hour via filters, water pumps or a combination of the two
Direct flow away from the roots (and avoid undergravel filters)
Plants only produce oxygen while lights are on; during the night cycle of your tank, they use oxygen too. This means you should use them as primary aerators only in lightly stocked tanks; otherwise add an air pump/air stone.
Freshwater River Habitat:
Add an inexpensive powerhead to your normal freshwater setup for a steady current
Position the powerhead so that a good percentage of your tank, especially hiding/resting places, are out of the current's path
Keep an eye on your fishes - do they enjoy playing in the current? Or do they seem to avoid it? You may need to adjust the flow to strike the right balance.
General Freshwater Tips:
Air stones are attractive, inexpensive, and promote surface gas exchange
Position pumps or powerheads to create maximum surface agitation
Saltwater Fish Only:
Turnover 5-10x/hour or more via in-sump pump or canister filter, and powerheads
Higher turnover helps waste processing if you keep large species
Attach powerheads at different spots of your tank to create turbulence, or purchase 1-2 powerheads with rotating returns - aerating (venturi) powerheads are great if your protein skimmer isn't air pump driven
Saltwater Reef:
Turnover 10x/hour (some aquarists suggest up to 20x or more) via in-sump water pump and powerheads
Don't aim current directly at invertebrates
Attach powerheads at different spots of your tank to create turbulence, or purchase 1-2 powerheads with rotating returns - aerating (venturi) powerheads are great if your protein skimmer isn't air pump powered
Wavemakers and water deflectors are excellent for mimicking random current patterns in reef tanks (with wavemakers, make sure your powerheads are compatible)
General Saltwater Tips:
Make sure any submersible devices you use are saltwater safe.
Avoid aiming currents directly at hiding spots or invertebrates.
Position powerheads for surface agitation, but avoid angles that produce salt spray, which can damage lights and other equipment near your tank's surface.
CONCLUSIONS
Whew! You made it. Hopefully you now feel confident in making circulation choices - experimenting with different setups can be rewarding as long as you keep in mind the key functions of healthy circulation. I'll leave you with some parting notes:
When investing in aquarium equipment, pay a little more for trustworthy name brands - you don't want this equipment to fail, and you want it to function well for a good long time, so don't take risks with quality that will come back to haunt you.
When it comes to GPH, estimate up and put a ball valve on your water pump to adjust down.
Two small pumps are better than one large pump.
If you aren't sure if you need an air pump, you can use an oxygen test kit once you get your setup running. Make sure you've got more than enough, to cover emergency scenarios
Whatever your main circulation method, it's cheap insurance to add a small air pump and air stones to prevent oxygen shut down during filter clogs or power failures.
Clean filters and pumps every 6 months, and change filter media as called for
Observe your tank daily - this is the fun part! A well circulated tank promotes growth and health. If you notice stalled growth or stress after increased or decreased flow, adjust as needed. If fishes are having trouble swimming or getting anywhere, lower your GPH.
Proper circulation, along with filtration and lighting, is a foundation of aquarium success, and it shouldn't be confusing or daunting to experiment with and find the proper setup for your aquarium. Hopefully you're now prepared to do just that!
We encourage readers who find this information beneficial to share it with friends or other interested parties. Link to this article from your own websites!
by Pearl A.
Functions of Circulation || Natural Water Movement || GPH || Filters
Air Pumps & Air Stones || Water Pumps || Powerheads & Wavemakers
Freshwater & Marine Scenarios || Conclusions
Water circulation is an essential and often misunderstood part of successful aquarium keeping. Some of the confusion is probably linked to the variety of devices that create circulation - water pumps, powerheads, filters, even air pumps. What do you actually need for your setup? How will different combinations of equipment affect those needs?
This article is here to help you understand aquarium water flow, and the equipment that creates it. We'll also discuss different circulation needs for different types of aquariums, so you can make smart choices when planning circulation or improving your setup.
FUNCTIONS OF CIRCULATION
Consider this example: the betta fish comes from small puddles or ponds, where it has adapted to breathe the air above the water's surface. This is why we can keep them, unlike other fish, in tiny bowls without circulation or filtration. But betta bowls need most or all of their water changed frequently, more frequently than larger tanks. If these fishes live naturally in stagnancy, why is this so important?
The unique isolation of a closed aquarium is significantly different from ponds or swamps, where stagnancy is offset by a great variety of plant and microbial life, changing weather conditions, and other dynamic processes. An aquarium is not a complete ecosystem, and the task of the aquarist is to create and maintain the stability and balance that occurs in nature.
Stagnancy has profound effects on a number of critical processes in the aquarium.
Gas Exchange
Imagine beating egg whites with a wire whisk. They become stiff, foamy peaks, because you're folding air into the whites by breaking their surface tension. That's how gas exchange occurs in water. Water movement creates the surface agitation needed to bring in oxygen, while beating out waste gases like nitrogen that need to escape.
Gas exchange occurs in still waters too, through the aquatic plants which take root where there is low water movement. They consume CO2 and release oxygen during the daytime (they use oxygen at night). They also consume nitrates, which would otherwise be converted into nitrogen gas by your biological filter. Thus the water's surface isn't the primary site of gas exchange in natural still waters.
However, in a planted aquarium you have other considerations (getting CO2 into your system in the day, getting oxygen into your system at night). And you'll still need to add minimal circulation to a planted aquarium to address some of the issues below. (More on planted tanks later.)
Oxygen and other Essential Elements
The amount of oxygen naturally dissolved into your water through surface gas exchange won't sustain livestock and the bacteria that process their waste, which use oxygen as well. Nor will surface movement distribute oxygen evenly throughout your tank.
"Dead spots" or anaerobic zones form when circulation is inadequate, especially in pockets of substrate and obstructed areas where flow can't reach. Dead spots are dangerous not only because they lack oxygen, but because the anaerobic bacteria that thrive there produce harmful chemicals. The same logic applies to other essential elements, like the trace elements needed by reef invertebrates, which rely on water movement to carry nutrients to them.
Filtration
Filters can't do much to clean and detoxify your tank if your water isn't moving through them a) completely and b) frequently. Even filters with pumps slow the flow of water as it passes through membranes and media inside. Since it's slowing down not once but every time it passes through, this can have a big effect when filters start to clog, especially if they are your only means of circulation.
Water Conditions
Proper water circulation is important to stabilize water conditions like temperature and salinity, where fluctuations can stress and even kill your organisms. Aquarium heaters and chillers affect only one area within your tank, relying on circulation to move water through this area for even heating or cooling. Salinity depends on dissolved salts, which with inadequate circulation can settle or "precipitate" creating false readings and potentially dangerous imbalances.
Algae
Normal algal growth is unavoidable, but discouraged by water movement. It can't form an ugly film on your water's surface, nor can it take hold on other surfaces as easily when water is constantly moving. In a planted tank, the natural competition for nutrients from other plants generally keeps algae in check. In other tanks, circulation can make the difference.
NATURAL WATER MOVEMENT
Underwater creatures have physiologically adapted to the way water flows in their natural environments. The importance of this is often underestimated, but when you think about it, makes a great deal of sense. Fishes used to strong currents are strong swimmers and moreover, need the exercise to maintain metabolic health, easily becoming overweight and unhealthy in still waters. Fishes unaccustomed to water movement lack the strength and stamina to deal with it, becoming stressed and even dying if forced swim constantly.
Adaptation to water movement is more pronounced when it comes to invertebrates. Corals, anemones, coralline algae, mollusks - these creatures developed to rely on vigorous water movement for all of their biological needs. Food and trace elements must be pushed through them, waste carried away; even spawning requires the help of currents. Plus, moving water cleans corals of sediment deposits that form in the carbonate rich waters of the reef - deposits which arrest growth, block feeding, dull colors, and generally impede good health.
It's not simply enough to observe how much water movement is present in the natural environment of your aquarium community. The type of water movement is important, and in fact, quite helpful in determining your setup.
Laminar currents - steady, uni-directional current found in rivers
Surge - strong current in one direction followed by weaker reverse current; found in oceans, it's the movement that provokes the characteristic back and forth motion of schools of fish
Turbulence - random water movement by multi-directional currents; found in oceans in combination with surge patterns
Lakes and Ponds - no strong current flow in one or any direction; even and gradual water movement.
We'll go over specific habitats in more detail towards the end of the article (or skip ahead if you'd like). First, with our new understanding of circulation, we can start breaking it down in terms of equipment.
GPH
GPH, or gallons per hour, is the unit used to measure water movement. Once you know how big your tank will be and what your turnover rate should be, you can calculate what GPH you need from your pumps and filters. For instance, if your tank is 50 gallons, and needs to be "turned over" 4 times in an hour, you'll need a pump rated 200 GPH (50 x 4).
Turnover rates vary to some degree, but here are a few general guidelines:
Tank Type Turnover Rate
Freshwater Fish-Only 4 times per hour
Planted Tanks 2 times per hour
Fry Tanks or Tanks with Small Freshwater Fishes or Bettas 2 times per hour
Freshwater Tanks with Large Specimens (Increased Waste) 5-6 times per hour
Saltwater Fish-Only 5-10+ times per hour
Saltwater Reef 10-20+ times per hour
That said, aim a little higher than you need. Adjust the turnover rate by 1 or look for the next highest GPH rating - this will help you overcome the resistance you encounter from equipment, filter media, plumbing fittings and dirt or clogging in any of those elements.
FILTERS
Different types of filters have different flow rates. Sometimes they will list their GPH; they may just have a tank size rating which indirectly refers to their flow rate. In smaller aquariums, particularly freshwater, you can achieve good circulation with filters alone.
The drawback of using filters as primary circulation is that you'll need to be quite vigilant about regular maintenance - not just in cleaning or replacing your filter media promptly, but also in cleaning the inside of your filters and tubing every few months.
Sponge filters, undergravel filters, and box or corner filters use air pumps to push water through them at a low flow rate. They may provide sufficient circulation for small tanks where very low flow is desired, like quarantine tanks and fry tanks.
External (or HOB - hang on back) power filters and canister filters have the highest GPH ratings. Internal power filters produce less flow than their hang-on counterparts. Power filters are often the primary source of circulation in smaller tanks.
Canister filters are pressurized - they sit outside your tank, so they have to move water further. If they don't come with their own water pump, they need one to provide that extra pressure - they may be powerful enough to circulate your tank, but look for one more powerful than you need to counteract any flow loss. Wet/dry filters, also outside or under your tank, require a water pump as well (and often don't include one).
AIR PUMPS & AIR STONES
Air pumps and stones aren't designed for circulation, though they do produce a little movement. Their bubbles (or micro-bubbles, in the case of airstones), do break the water's surface tension to aid in gas exchange. But these streams of air cannot move your entire volume of water, much less evenly. This explains the lack of GPH ratings on air pumps - they tell you how much air they produce (or how large of a tank they aerate).
Therefore, an air pump/filter combo may be perfectly sufficient for small or shallow tanks, but once you get above 50 gallons, you'll need a water pump to circulate oxygen (barring a powerful pump-driven filter). Some water pumps aerate too, but it's good practice to backup aeration from filters or pumps - in case of clogs or power loss, air stones will keep going, and can save lives.
WATER PUMPS
Water pumps move water without strong currents, ideal for even circulation. Many experienced aquarists recommend 2 less powerful water pumps over 1 pump that meets your total need - this way, if one malfunctions, you have the other to buy you time to fix or replace the first. Water pumps are the best way to ensure total movement of a large tank - if you need directional flow but still want the circulating power of a normal pump, combine it with a powerhead (more in the next section).
Head Height
You will see charts in pump descriptions listing "200 GPH at 0 head height/150 GPH at 3 feet", and so on - with GPH decreasing as head height increases.
Head height is the distance from the pump to the highest point it must reach (not necessarily the surface of your tank, but wherever the output of the pump will feed to). If your head height is 0, your pump is flush with its output flow. If you'll be keeping your pump in a sump, you may have 5-6 feet of head height. All pumps will provide head height/GPH correlations, so as long as you reasonably estimate your head height you can simply refer to the GPH rating at that height.
Resistance & Plumbing
Plan on resistance from equipment and circumstances likely to slow your flow rate. A good rule is to estimate about a step up from the recommended GPH. If you're using canister filters, which are pressurized, or will need extensive plumbing, take it one step further (turn your tank 6 times instead of 4). You can always adjust your pump's GPH down, but never up, so err on the side of more power.
You will see measurements for input and output tubing in the specifications for any pump. Never use input tubing smaller than the input specifications. Some people use smaller output tubing to decrease their output flow. This strains both your tubing and your pump and results in shortened life or more immediate problems. If you want to decrease your output, attach a ball valve, which gives you greater control of adjustment anyway.
Submerged v. External
Submerged pumps are quiet and easy to install. They will produce some heat, and if you have a saltwater tank make sure any submerged equipment is saltwater safe. Often these pumps are placed in a sump, minimizing heat and clutter in your tank. In this case, you'll need to match the dimensions of your sump's pump chamber.
External pumps won't heat your tank as much, but you'll need somewhere to put them, and they can be noisier (some brands feature quiet operation, look for those if it's a concern). Usually they're configured either for high pressure use or circulation (free-flow). Pressure setups are needed if you're installing the pump in-line before your canister filter, feeding water at high pressure, or if you're operating at extreme head heights. Free flow pumps are for general circulation or returning water to your tank from your wet/dry sump.
External pumps may require modifications to your sump or filters to accommodate plumbing. However, some aquarists don't like using submerged pumps with reef systems - the calcium or kalkwasser solutions that are dosed in these systems can lead to more pump cleaning and possible lessened efficiency.
POWERHEADS & WAVEMAKERS
Powerheads are small, submersible water pumps which attach to the walls of your tank and produce laminar, or uni-directional, currents. They make a healthy addition to a river habitat, providing the right style of current for muscular river fishes to get a good workout. Powerheads are also some of the best tools for saltwater aquariums. They are inexpensive, so you can buy 2 or even more and place them at different points on your tank to replicate ocean turbulence.
There is a lot of variety among powerheads - some aerate, some have rotating returns, others can function as normal pumps. Because they are smaller and uni-directional, it's better to use 2 than 1 for circulation. You don't want to aim a powerhead straight at the popular hang-out spot in your aquarium, or directly at invertebrates - it's nice for organisms to be able to take refuge from the strong movement if they tire, and corals thrive along currents rather than in their path. (You can hand-hold a powerhead to blast particularly tough detritus from the surfaces of corals, if problem areas form.)
The other benefit of powerheads for saltwater aquariums is that some models can be attached to wavemakers. Wavemakers are basically power strips with outlets designed to alternate electrical current, switching power at intervals between powerheads to create movement close to random alternating currents. This type of vigorous water movement is ideal for reef tanks.
FRESHWATER & MARINE SCENARIOS
Here are some setup suggestions for scenarios you may encounter. With basic plumbing skills, you can design custom setups with any flow directionality you can dream up, but we'll stick to commonly used equipment here.
Small Freshwater Bowls/Tanks up to 3 gallons:
Biweekly water changes of 20% or more
No additional circulation
Small Fry Tanks, Quarantine Tanks and Tanks with Small or Pond Fishes:
Turn over 2x/ hour
For fry and quarantine tanks, an air pump-driven sponge filter provides the circulation you need without damaging fish or removing medicines.
For small fish tanks or tanks with betta or goldfish, an internal corner or box filter, also air pump-driven, can be used instead if preferred, although a sponge filter is still a good choice. Look for an air pump with multiple outlets, so you can add an air stone for improved surface gas exchange.
Freshwater Fish Only Tanks:
Turn over 4x/hour
In tanks with large specimens, higher GPH (5 or 6x/hr) assists with increased waste.
Power or canister filters alone can circulate tanks up to 50 gal or so; add an air pump & airstones to avoid oxygen shut down in case of filter clogs or power outages.
Add an aerating water pump or a water pump/air pump combo to tanks larger than 50 gallons to prevent anaerobic dead spots.
Freshwater Planted:
Turn over 2x per hour via filters, water pumps or a combination of the two
Direct flow away from the roots (and avoid undergravel filters)
Plants only produce oxygen while lights are on; during the night cycle of your tank, they use oxygen too. This means you should use them as primary aerators only in lightly stocked tanks; otherwise add an air pump/air stone.
Freshwater River Habitat:
Add an inexpensive powerhead to your normal freshwater setup for a steady current
Position the powerhead so that a good percentage of your tank, especially hiding/resting places, are out of the current's path
Keep an eye on your fishes - do they enjoy playing in the current? Or do they seem to avoid it? You may need to adjust the flow to strike the right balance.
General Freshwater Tips:
Air stones are attractive, inexpensive, and promote surface gas exchange
Position pumps or powerheads to create maximum surface agitation
Saltwater Fish Only:
Turnover 5-10x/hour or more via in-sump pump or canister filter, and powerheads
Higher turnover helps waste processing if you keep large species
Attach powerheads at different spots of your tank to create turbulence, or purchase 1-2 powerheads with rotating returns - aerating (venturi) powerheads are great if your protein skimmer isn't air pump driven
Saltwater Reef:
Turnover 10x/hour (some aquarists suggest up to 20x or more) via in-sump water pump and powerheads
Don't aim current directly at invertebrates
Attach powerheads at different spots of your tank to create turbulence, or purchase 1-2 powerheads with rotating returns - aerating (venturi) powerheads are great if your protein skimmer isn't air pump powered
Wavemakers and water deflectors are excellent for mimicking random current patterns in reef tanks (with wavemakers, make sure your powerheads are compatible)
General Saltwater Tips:
Make sure any submersible devices you use are saltwater safe.
Avoid aiming currents directly at hiding spots or invertebrates.
Position powerheads for surface agitation, but avoid angles that produce salt spray, which can damage lights and other equipment near your tank's surface.
CONCLUSIONS
Whew! You made it. Hopefully you now feel confident in making circulation choices - experimenting with different setups can be rewarding as long as you keep in mind the key functions of healthy circulation. I'll leave you with some parting notes:
When investing in aquarium equipment, pay a little more for trustworthy name brands - you don't want this equipment to fail, and you want it to function well for a good long time, so don't take risks with quality that will come back to haunt you.
When it comes to GPH, estimate up and put a ball valve on your water pump to adjust down.
Two small pumps are better than one large pump.
If you aren't sure if you need an air pump, you can use an oxygen test kit once you get your setup running. Make sure you've got more than enough, to cover emergency scenarios
Whatever your main circulation method, it's cheap insurance to add a small air pump and air stones to prevent oxygen shut down during filter clogs or power failures.
Clean filters and pumps every 6 months, and change filter media as called for
Observe your tank daily - this is the fun part! A well circulated tank promotes growth and health. If you notice stalled growth or stress after increased or decreased flow, adjust as needed. If fishes are having trouble swimming or getting anywhere, lower your GPH.
Proper circulation, along with filtration and lighting, is a foundation of aquarium success, and it shouldn't be confusing or daunting to experiment with and find the proper setup for your aquarium. Hopefully you're now prepared to do just that!
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