Wendy: Here is a good article that provides some good guidance on the proper and types of flow:
It is my opinion that next on the list of importance after water quality and aquarium lighting is water circulation. Before deciding upon the method to create water circulation in the aquarium it is important to understand why water circulation is so important as well as the different types of water flow.
Circulation within the aquarium is important for various reasons. It allows for the provision of food to the corals as well as the removal of waste. Another reason is that it allows for the provision of oxygen. Cold water does not have the ability to be able to hold as much oxygen as air does, warm water even less and warm saltwater even less than that. The movement of water allows for what is called an air/water interface to occur. This is basically when water is is brought into contact with the air and oxygenation of the water can occur.
In nature every animal is accustomed to varying strengths of water movement and throughout the years life has adapted to the various types of water movement. Corals are a perfect example of this with each different species of coral having different requirements when it comes to water movement. It is for this reason that it is imperative that corals are placed in an area of the aquarium where there water movement requirements, as well as lighting requirements are met.
What about flow and how much is enough?
There is a very general rule of thumb which is used when it comes to the question of how much flow and this is dependent upon the type of corals being kept. If soft corals are being kept in the aquarium then flow should be provided which is ten times the net water volume of the aquarium. If hard corals are being kept then water flow should be provided which is in the region of thirty times the net water volume of the aquarium.
As said this is a general rule of thumb and is primarily based upon a minimum requirement. It is not uncommon for reef tanks to have water flow in the region of sixty times the net water volume or more.
It is easy to provide flow to any aquarium but it is important that the correct type of flow is provided. There are basically three types of flow.
Laminar Flow
Laminar flow is where the water flow all goes in one direction i.e. from one end of the aquarium to the other. This type of water is the standard type of flow which is provided by a powerhead.
Turbulent Flow
Turbulent flow is where the flow is random. All of the water is travelling in multiple directions.
Surge Flow
Surge flow is where the water flow moves from side to side in a back and fro motion.
Out of these three flow types the only one which is not desirable is laminar flow. Surge flow is good however the best type of flow in a reef aquarium is turbulent flow. With this type of flow water will be moving in all directions and there should be no dead spots in the aquarium.
When providing water flow to the aquarium you need to ensure that there are no dead spots. All water in the aquarium should be moving to some extent or another
Another area which is quite often overlooked by many aquarists is the size of the flow. On a standard powerhead the outlet of the device is normally quite small which therefore creates flow which is very much like a jet. Whilst a lot of water can be moved with this size outlet the force of the water can be damaging. Other devices have outlets which are very wide. These devices provide water flow which is wide and very soft. The benefit of the latter one is that a huge amount of water can be moved around the aquarium and corals etc will not be damaged by the flow.
The creation of water flow is fairly simple and there are various devices and/or techniques which can be used to perform this.
Power Heads
Powerheads are relatively inexpensive and are avaialable from various stores both on the internet and from local stores. The majority of these devices use impellors to draw water into their housing and force it out of the outlet. As mentioned above a lot of these devices have small outlets and there create water flow which is forceful. There are, however other devices which have larger outlets and are normall driven by propellers rather than impellors to move the water. Due to the large outlet more water can be moved and it is not as forceful. Power heads by their design create water flow which is laminar. It is therefore important to ensure that all attempts are made to locate powerheads in varying areas of the aquarium where the flows collide or bounce off the aquarium glass to make the flow more turbulent. This can take time to achieve however with patience good flow can be created.
Oscillating Devices
There are a few types of oscillating devices available on the market at the moment. Some of these devices are similar in design to that of the powerhead however the outlet areas moves around therefore creating varying laminar flow. When combined with powerheads or with other oscillating devices the flow can be good.
Other types are devices which attach to pumps, powerheads etc and allow the flow to be alternatated between outlets therefore providing another element of random flow into the aquarium.
Wave Makers
There are a few types of wave maker on the market at present. This first is like a power strip where all the powerheads are plugged into it. This devices stops and starts the powerheads at varying intervals which allows for a random flow to be created. It does need to be noted that with this type of device the powerheads used should have soft start motors in them. Powerheads are not designed to be turned on and off every X amount of minutes and therefore more wear is created on the motor and the impellor.
Another type of wavemaker is very similar in design to that of a wave maker in a swimming pool. These are normally box like devices where there are two holes. A motor drives the unit and water is taken into one of the holes and is blown out of the other. The motor then reverses the process and water is blown out of the other hole and sucked in the other. This type of device creates excellent flow within the aquarium however quite often they are bulky items.
Surge Devices
In the past surge devices were quite noisy and introduced a large amount of bubbles into the aquarium. These devices were items like dump buckets etc. Technology has now moved on and there are devices which are similar in design to that of a powerhead however electronic devices allow the unit to run at full power for X seconds and then reduce in power to say 25% for another X seconds and then vice versa. These types of devices normally have wide outlets and therefore the flow is soft yet strong. If two of these devices were installed into the aquarium with one being at each end with each device alternating its output then the water flow created can be excellent.
Another device which falls under this category is one which is called a wave box. This type of device uses an empty box with a power head at the bottom. When the box is empty the powerhead turns off allowing water to fill the box. When the box is full the powerhead is activated which expels the water in the box. Excellent water flow can be created with this device especially when combined with another type of device.
Closed Loops
Closed loops are where the water pumps used for moving water about is located externally to the aquarium. Pipework is attached to both the inlet and the outlet of the pump and both the outlet and inlet are submerged underwater either via the pipework itself or drilled holes in the aquarium with bulkhead fittings attached. When the pipe is filled with water (primed) and the pump turned on there is no head loss experienced on the pump as the water does not have to fight against gravity. Benefits of this type of devices are that the physical unit is removed from the aquarium which therefore means that a heat source is removed as well as a large unsightly item or items. It does need to be noted that a closed loop system does need careful planning and is more suited to aquariums which are in the process of being setup. This does not mean that closed loops can't be used on an existing aquarium as they can but they are slightly harder to install.
With all of the devices listed above it is important to note that the correct placement of the device is the secret to its success. The best example I can use for this is the powerhead. If the aquarium in question has a requirement for 1500gph of water movement and three powerheads are purchased to facilitate this requirement then it is not simply a case of installing the powerheads into the aquarium. The powerheads need to be placed in the aquarium so that the flow provided by the powerheads clash with each other, bounce off the glass, bounce off the rocks etc. They could be provided at the rear of the aquarium facing forwards or at slight angles to each other.
The placement of closed loop systems is slightly trickier is you have to attempt to hide the pipework. A single closed loop is powered by one pump however the aquarium can have more than one loop in the aquarium. Each loop, dependent upon the size of the tubing used can have more than one outlet and these outlets can be placed in numerous placed in the aquarium. You could have two in the top from corners of the aquarium, two in the front bottom of the aquarium other outlets hidden in the rockwork. The pipe at the bottom of the aquarium could be hidden by sand etc, the one at the top could be hidden on the aquarium strapping and the others hidden down the rear of the aquarium. The installation of outlets using closed loops is endless - the hard thing is hiding the pipework!
Every aquarium is different due to size, rockwork layout, corals etc. The placement of the device therefore requires careful consideration as well as a bit of fiddling.
Closure
As mentioned above the creation of water flow is imperative in any reef aquarium. Not just any flow but the right type of flow and the right amount of flow. This can be created in various ways and it is the responsibility of the aquarist to ensure that the method chosen is correct for the aquarist themselves as well as their individual aquarium and their aquarium inhabitants requirements.
