Are Deep Sand Beds, DSBs, dangerous to use in a marine aquarium?

[Trigeek]

I have used a DSB in my tank for the past eight years. I changed the sandbed gradually over a three month period this past spring. Let me tell you, the sand I pulled out was filled with gunk. Totally gunkified. Horrifying. :hmm3:

I suspected this, which is why I changed the sand. I'm using different husbandry now. I siphon from the sandbed on water changes, doing a different part of the bed each week. I know where the low flow zones are and where the crap accumulates most and go there for siphoning. Able to work about a 1/3 of the sand for a 15 gallon water change. At the end of the effort, the water pulled from the sand is clear (not cloudy brown detritus).

Every 4-6 weeks, I move the rocks on part of the tank and clean there. This is labor intensive, though not too bad really. I feel this process is necessary for mantenence of a tank with sand and am not willing to just let the crap sink into the sand and stay there.

I'm working on developing a "sand cleaner consisting" of an acrylic globe drilled to allow for two hoses from a filter (diatom probably or strong canister?) which I'll use on part of the sandbed 1-2 times per year.

I have a ton of critters in my tank (pods, worms etc.). The populations don't diminish even with my vacuuming as the startegy is to do one part of the bed at a time. Replacing the sand bed every so often is necessary too. I like the sand for aesthetics, biodiversity, sand dwelling fish etc. but it definately needs to be attended to. We'll see how this startegy does over the long haul.

 

[simonh]

I've had DSBs, non-DSB, tried carbon dosing etc. etc. I'd say NO. I don't see a plethora of tanks without DSBs that are lasting significantly longer than those with DSB's. I also don't agree with the statements about P sinks etc.

There are lots and lots of tanks on RC (or TOTM) around the 2-3 year mark that are amazing using allsorts of methods. There really aren't many around the 10 year mark to make judgements about.

 

[Reefin' Dude]

I've had DSBs, non-DSB, tried carbon dosing etc. etc. I'd say NO. I don't see a plethora of tanks without DSBs that are lasting significantly longer than those with DSB's. I also don't agree with the statements about P sinks etc.

what other information would you like, that was not presented in this thread? phosphate binds to calcium carbonate. there is not an export model available for phosphates to leave a marine substrate. phosphates are mined from old sea beds. how does all of this not point to substrates being phosphate sinks. whether they are chemically P sinks, biological P sinks, or a combination of the two. they sink nutrients. all you have to do is stir up any substrate to prove this. any material within the substrate that is not calcium carbonate and not put there by you will have nutrients in it. if more material is accumulating, then the total nutrient level of the substrate must be increasing. the fact that there are organisms in the substrate feeding on the material just means more nutrients in the substrate, or they would not be there, no food.

there is an entire ecosystem all into itself in a substrate. the problem is that it is fed from the top and without a "bottom" that can remove the wastes. think of the substrate as a single organism if it helps. it needs to be fed, and it needs to be cleaned up after. the hard part is that our tanks have a solid bottom and makes it difficult to remove the waste that has worked its way down to the hard bottom. there can be a huge biomass in the substrate, but as long as there is more material entering it from above, then there is going to an ever increasing amount of nutrients with in the substrate. a sink.

G~

 

[Saltliquid]

The way most people put in substrates is the reason they can slightly or greatly pollute and of course, the lack of maintenance.
If the developing sink is below the media, then you have very little issues, only beneficial issues.
Put in a raised floor, a deep sand bed or any substrate anywhere in your waters is a mistake with out one!
When that low PH combines with calcium media and phos, the precipitation is only good for clogging and nothing else.
The best one I have made was with a raised floor for the media and glass boxing where the live rock was to sit on the substrate at the height of the planned substrate with a slight sprinkling of sand on top of that sealed glass boxer area.
Live rock on the substrate is bad as you can not affectively siphon under your rock with out moving it and slightly destabilising your many bacterium and subsequent PH, oh and release some Hydrogen sulphide to upset the your nitrate reducing bacteria.

 

[simonh]

phosphate binds to calcium carbonate. there is not an export model available for phosphates to leave a marine substrate.

It is true that there is no gas phase for phosphates to leave the tank unlike nitrate which can be reduced to nitrogen gas and escapes to the atmosphere. The sand bed acts more like a nutrient rearrangement area, the nutrients are rearranged into biomass of the animals in the sand bed and spawning to feed other animals in the tank.

Calcium carbonate can bind phosphates but that is generally in equilibrium with the surrounding water. It is not some endless sink unless the calcium carbonate crystal was growing and trapping it within the matrix (in which case it would be unlikely to be a concern anyway).

I also think you are ignoring the fact that organisms in the sanded consume detritus and use it to either grow or reproduce. Most reproduction into the water column would be an export of those nutrients as food for corals or be removed by other filtration such as skimming.

Sure some people run sand beds without success. Many never have any real population of organisms to reorganise the nutrients into something useful. Others have predators that don't allow the sanded fauna to become established. Some people have far too many animals in the tank and the amount of food going into the system overwhelms the processing capabilities. However, most of this is true of other methods which also fail.

 

[Paolo Piccinell]

I also think you are ignoring the fact that organisms in the sanded consume detritus and use it to either grow or reproduce. Most reproduction into the water column would be an export of those nutrients as food for corals or be removed by other filtration such as skimming.

Sure some people run sand beds without success. Many never have any real population of organisms to reorganise the nutrients into something useful. Others have predators that don't allow the sanded fauna to become established. Some people have far too many animals in the tank and the amount of food going into the system overwhelms the processing capabilities. However, most of this is true of other methods which also fail.

I do agree completely!!! :beer:

When a DSB crashes, 99% of the times it is due to an aquarist's fault.
Overpolulation, overnutrition, predators of infauna incautiously added.

DSB is a nutrient SINK, it is absolutely true.
But it is not a problem for the aquarium system... it is a problem if DSB becomes a nutrient SOURCE, but it depends on the aquarist's wrong expectations about the quantity of nutrients/detritus a DSB can manage.


Nutrients sink in the sand in form of detritus and emerge as new life forms (bacteria, monocellular algae, benthic and planktonic larvae, etc.) that feed the aquarium animals (sponges, idrozoans, corals).

 

[power boat jim]

I do agree completely!!! :beer:




Nutrients sink in the sand in form of detritus and emerge as new life forms (bacteria, monocellular algae, benthic and planktonic larvae, etc.) that feed the aquarium animals (sponges, idrozoans, corals).

As much as some would like to believe that sand is magic, this is simply not possible. Poop and uneaten food does not mysteriously transform into a new life form that is beneficial to a tank. Any animal that consumes detritus will itself leave a waste product behind. There will always be biomass left in the sand that accumulates. Thats the part that needs to be removed from the sand from time to time.

 

[Paolo Piccinell]

As much as some would like to believe that sand is magic, this is simply not possible. Poop and uneaten food does not mysteriously transform into a new life form that is beneficial to a tank. Any animal that consumes detritus will itself leave a waste product behind. There will always be biomass left in the sand that accumulates.

Absolutely right.
But each trophic level leaves less detritus than the previous one.

So, realising a complex food net in the aquarium is the key to success with DSB, together with a nutrient removal device, as a skimmer, that catches the leftovers after all the creatures housed in the tank had their dinner.

 

[power boat jim]

Absolutely right.
But each trophic level leaves less detritus than the previous one.

So, realising a complex food net in the aquarium is the key to success with DSB, together with a nutrient removal device, as a skimmer, that catches the leftovers after all the creatures housed in the tank had their dinner.

In theory this would work, but it is not possible to get 100% of the detritus suspended and into a filter on its own. This is why the bed needs, in my experience anyway,need to be stirred or siphoned to dislodge the build up that stays in the sand.

 

[Paolo Piccinell]

Well, the detritus that cannot be suspended and filtered remains buried in the sand bed.

If an organic substance passes through different intestines for three/four times, there remains very few organic pollutant.
The ultimate leftovers just remain buried in the sand bed, that's why I do not disturbe it.


(Please forgive me for my terrible english... I lack a lot of terminology, but I hope my way of thinking is clear)

 

[power boat jim]

Well, the detritus that cannot be suspended and filtered remains buried in the sand bed.

If an organic substance passes through different intestines for three/four times, there remains very few organic pollutant.
The ultimate leftovers just remain buried in the sand bed, that's why I do not disturbe it.


(Please forgive me for my terrible english... I lack a lot of terminology, but I hope my way of thinking is clear)

What ever is left over is still mostly all organic, even after it passes through intestines. It is still biomass capable of producing H2S upon decay if conditions are favorable for it.

Your english is very good, better them some here.

 

[Paolo Piccinell]

H2S production is a remote eventuality... really remote.

The smell of rotten egg is detected by human nose so well that before reaching a dangerous concentration for aquarium, You will notice it and will be able to react.

IMHO, obviously

 

[power boat jim]

H2S production is a remote eventuality... really remote.

The smell of rotten egg is detected by human nose so well that before reaching a dangerous concentration for aquarium, You will notice it and will be able to react.

IMHO, obviously

All that is needed for H2s production is an anerobic state and organic material. Its not that uncommon of a condition created if you have sand 4'' or so deep. Its simply the result of anerobic digestion.

On a side note that has little to do with the reef tanks we keep, one of the first effects H2S has on the body is deadining our sense of smell. Thats why H2S is so dangerous to people who work in environments where this gas could be present. Not that you could ever reach that level in a reef tank , just being aware that it does occur in other situatuions may be of use.

 

[Reefin' Dude]

It is true that there is no gas phase for phosphates to leave the tank unlike nitrate which can be reduced to nitrogen gas and escapes to the atmosphere. The sand bed acts more like a nutrient rearrangement area, the nutrients are rearranged into biomass of the animals in the sand bed and spawning to feed other animals in the tank.

it is still sinking the P. whether it is inorganic or organic does not matter, the P is still there and increasing. if there is more life, than there is more P to feed the additional biomass. if not P in the substrate, than there would not be any biomass in the substrate, no food, no critter. you can not have a growing biomass without a growing food source to feed the biomass.

Calcium carbonate can bind phosphates but that is generally in equilibrium with the surrounding water. It is not some endless sink unless the calcium carbonate crystal was growing and trapping it within the matrix (in which case it would be unlikely to be a concern anyway).

not completely, but as you said in the previous bit, it is a rearrangement of P. the P attaches to the calcium carbonate, lets say equilibrium to keep it simple. the P then gets released from the calcium carbonate by the bacteria, because it is easier for them to do. now the P is in the bacteria. the bacteria then die, or get consumed by other bacteria or other organisms, etc"¦ ad infinitum, or whatever. this is all going on in the substrate. each layer of substrate creates a semipermeable barrier, making it harder for any P to equilibrate with the actual water column. as one goes deeper the amount of inorganic P goes up, and the amount of food for all of the biomass in the substrate. at some point other resources can become problematic and the process can stop. lack of N, O, or easily available C.

I also think you are ignoring the fact that organisms in the sanded consume detritus and use it to either grow or reproduce. Most reproduction into the water column would be an export of those nutrients as food for corals or be removed by other filtration such as skimming.

i think you are ignoring the fact that in order to have a growing population of any organism, there has to be a growing source of food. if not than the population would die. if the population would die, then what would happen to the P that is or was in this biomass?

Sure some people run sand beds without success. Many never have any real population of organisms to reorganise the nutrients into something useful. Others have predators that don't allow the sanded fauna to become established. Some people have far too many animals in the tank and the amount of food going into the system overwhelms the processing capabilities. However, most of this is true of other methods which also fail.

we need to think of substrates as an entire organism, not a bunch of them. something that needs to be fed and cleaned up after in order for it to maintain its health.

ecosystem is just a fancy work for phosphate sink.

I do agree completely!!! :beer:

When a DSB crashes, 99% of the times it is due to an aquarist's fault.
Overpolulation, overnutrition, predators of infauna incautiously added.

absolutely, but wouldn't the more organisms the more steps, the harder it would be to maintain consistency in the system? the more links in the food chain the more chances that a chain could crash from one of the examples listed above. all of these links are for what? why not just remove the detritus and just feed and keep the organisms you really want to spend money on?

DSB is a nutrient SINK, it is absolutely true.
But it is not a problem for the aquarium system... it is a problem if DSB becomes a nutrient SOURCE, but it depends on the aquarist's wrong expectations about the quantity of nutrients/detritus a DSB can manage.

absolutely, a substrate must be fed and cleaned up after, or it will become a nutrient source back into the system. from lets say P equalization because the P can not longer be slowly migrated downward because all available space between the sand has become full of detritus. resources are so used up or bound and the only available outlet is upwards.

Nutrients sink in the sand in form of detritus and emerge as new life forms (bacteria, monocellular algae, benthic and planktonic larvae, etc.) that feed the aquarium animals (sponges, idrozoans, corals).

it can not work that way. all of these organisms that are living in the substrate need to feed on something, they also much poo. granted there maybe some P exported through this method, but if more was being exported than imported into the substrate, there would be less life in a substrate and not more.

i have also not found any P cycle graphics that show P exported from a marine substrate besides geologic upheaval.

G~

 

[simonh]

The food webs are going to be dynamic. When more food is available the populations will expand, when less becomes available some organisms will die and the rest of the population will feed on that released P until it finds an equilibrium once again.

each layer of substrate creates a semipermeable barrier, making it harder for any P to equilibrate with the actual water column. as one goes deeper the amount of inorganic P goes up, and the amount of food for all of the biomass in the substrate.

So, the P is locked deep in the substrate?

a substrate must be fed and cleaned up after, or it will become a nutrient source back into the system.

OK, so now the P is been released back into the system.

So what happens to your sink?

Is it a sink that stuff goes into and never comes out of? That would be good!
Is it a sink that gets blocked and something catastrophic happens? (no one seems to ever state what? P release? H2S?) Under what conditions?
Is it a buffer that stuff is sinked when there is an excess and then released when the water column has less than the sandbed?

I've never seen a good explanation about what happens to this sink. I'd like to hear the proposed mechanism of what turns this sink bad?

 

[Reefin' Dude]

The food webs are going to be dynamic. When more food is available the populations will expand, when less becomes available some organisms will die and the rest of the population will feed on that released P until it finds an equilibrium once again.

yes, but where did the food come from in the first place if it was not there when you put the sand into the system? if the substrate worked nutrients upwards instead of downwards, then there would never be a build up of any detritus in the substrate over time. we could all just swish around the substrates whenever we want without any fear of H2S or nutrient release, correct?

i can not remember which threads i have posted these graphics, but it wasn't this one apparently.

So, the P is locked deep in the substrate?

correct. i have not found a P cycle graphic that shows otherwise. if you find one please post it.

OK, so now the P is been released back into the system.

yes, it can not accept any more P. the flow of equilibrium is now not into the sink. the slow migration of P downward stops.

So what happens to your sink?

it needs to be cleaned of detritus, and then the process starts all over again.

Is it a sink that stuff goes into and never comes out of? That would be good!

that would be nice, but unfortunately our tanks have a hard bottom. the detritus can only migrate so far before it reaches the tank bottom and starts to pile up. the detritus is bacterial flock.

Is it a sink that gets blocked and something catastrophic happens? (no one seems to ever state what? P release? H2S?) Under what conditions?

no, nothing catastrophic can happen. the system just moves up a trophic state. instead of being lets say oligotrophic, it becomes eutrophic. whether or not the organisms in the system can handle the change in trophic state determines if there is a "crash" perceived by the aquarist.

Is it a buffer that stuff is sinked when there is an excess and then released when the water column has less than the sandbed?

yes, it is P buffer if you will. in the beginning nearly all of the P is going into the substrate, then over time less can go into the substrate, more stays in the water column until very little can go into the substrate, and all incoming P to the system just stays in the water column. as more P becomes available the more algae shows up, the more measures needed to counter these affects. after the fact P removers. GFO, ATS's, live sumps, carbon dosing, stronger lights, Ca reactors, the list goes on. these are devices used to counter the affects of a system becoming more eutrophic.

I've never seen a good explanation about what happens to this sink. I'd like to hear the proposed mechanism of what turns this sink bad?

they turn bad in the same was as a septic tank turns bad, or any filter that can no longer hold anything more.

G~

 

[simonh]

The ecosystem pictures are great. Unfortunately, I'm missing a number of components such as the rabbit ! I also don't have the deep sea where nutrients may sink to and become isolated from the reef and locked away. A sand bed is at most a few inches from the bulk of my water column!

then there would never be a build up of any detritus in the substrate over time. if the substrate worked nutrients upwards instead of downwards,

There is always going to be some 'detritus' in whatever state of decomposition. Some tied up in organisms. Not all detritus is going to be loaded with N/P depending how many organisms it has passed through.

Why would the in fauna be working all detritus / nutrients down and not moving them through the sand at different levels with some even been brought back to the surface and out even.

So, the P is locked deep in the substrate?
correct.

Phew. So if it is locked deep in the substrate I don't need to worry about that P. ! It's a one way SINK. I never need to worry.

OK, so now the P is been released back into the system.

yes, it can not accept any more P. the flow of equilibrium is now not into the sink. the slow migration of P downward stops.

This contradicts the P been locked deep in the substrate. If the P can be released into the water column "when full" due to equilibrium then the P can be released up from deep down into higher layers by equilibrium too. At most we have a gradient. We are talking 6" here not deep sediments and geological cycle shown in the P diagram. The diagram is great on a macro level, but it doesn't illustrate whats happening on the micro level between a shallow substrate and the water column (I'm sure on the diagram a shallow substrate is probably much greater than the deepest of DSBs).

yes, it is P buffer if you will. in the beginning nearly all of the P is going into the substrate, then over time less can go into the substrate, more stays in the water column until very little can go into the substrate, and all incoming P to the system just stays in the water column. as more P becomes available the more algae shows up, the more measures needed to counter these affects.

So this is no different to if I have a bare bottom tank. Any detritus that drops to the bottom of the tank and is not cleared up quickly starts releasing P into the water column and causes algae etc. A sand bed at worst gives me a buffer... while I'm not taking too much care of my tank the detritus and P builds up in the tank and substrate, once I start to take better care the P starts to come into equilibrium with the tank water and begins to be removed from the substrate.

after the fact P removers. GFO, ATS's, live sumps, carbon dosing, stronger lights, Ca reactors, the list goes on. these are devices used to counter the affects of a system becoming more eutrophic.

I've never heard of a calcium reactor been required because the system has become eutrophic. They are usually used in oligotrophic systems with great SPS growth.

 

[Reefin' Dude]

The ecosystem pictures are great. Unfortunately, I'm missing a number of components such as the rabbit ! I also don't have the deep sea where nutrients may sink to and become isolated from the reef and locked away. A sand bed is at most a few inches from the bulk of my water column!

and that is the problem. the marine environment is missing the true plants, which feeds the rabbit. that is why there is a split between the marine side and the land based side. you do have the deep sea, it is the substrate. in our systems that is the P sink.

There is always going to be some 'detritus' in whatever state of decomposition. Some tied up in organisms. Not all detritus is going to be loaded with N/P depending how many organisms it has passed through.

if it does not contain N and P, then what does it contain? anything that can be used as food for another organism must contain N and P.

Why would the in fauna be working all detritus / nutrients down and not moving them through the sand at different levels with some even been brought back to the surface and out even.

gravity is a b****. the only upward movement could be from an equilibrium reaction, but the more material in the way the harder it is for this equilibration reaction to occur. the more material the greater the difference between the layers.

Phew. So if it is locked deep in the substrate I don't need to worry about that P. ! It's a one way SINK. I never need to worry.

correct, you can ignore it until the sink becomes full, then all of the P that was going into it can not and all of the P now stays in the water column. just like you never have to worry about changing the oil filter on your car.

This contradicts the P been locked deep in the substrate. If the P can be released into the water column "when full" due to equilibrium then the P can be released up from deep down into higher layers by equilibrium too. At most we have a gradient. We are talking 6" here not deep sediments and geological cycle shown in the P diagram. The diagram is great on a macro level, but it doesn't illustrate whats happening on the micro level between a shallow substrate and the water column (I'm sure on the diagram a shallow substrate is probably much greater than the deepest of DSBs).

correct. there is a gradient, and that is why the substrate acts like a buffer up until it gets full. the finer the substrate the greater the gradient can be. the more coarse the less gradient and the better the equilibrium can be maintained. this of course means that affects of the sink filling up will be seen faster. as is seen by people who use coarse substrates instead of fine substrates that follow the same maintenance routines.

So this is no different to if I have a bare bottom tank. Any detritus that drops to the bottom of the tank and is not cleared up quickly starts releasing P into the water column and causes algae etc. A sand bed at worst gives me a buffer... while I'm not taking too much care of my tank the detritus and P builds up in the tank and substrate, once I start to take better care the P starts to come into equilibrium with the tank water and begins to be removed from the substrate.

exactly. it is all about determining where you stand on the maintenance/cost curve for the given environment you want to emulate. you can get the detritus out constantly, or let is sink, then get it out when it becomes a problem later. of course it is not that simple when we take into accounts the different trophic environments we want to emulate. some corals we keep like more P, others find it toxic. the amount of P sink can be helpful depending on the environment wanting to be emulated.

do you really want the P to be in equilibrium with the water? by the time this occurs you are already behind the curve. we would rather have the higher P in the water and the lower P in the substrate at the boundary layer. luckily the bacteria make this happen for us. the best way to go after N and P is by removing the detritus before it can become a nutrient source for bacteria. it makes more sense to me anyway to use the substrate P sink at what it does best, bind P. you remove the detritus from the substrate on a regular basis (based on the trophic level wanting to be emulated, this could be weekly or after several years, or nearly never) allowing the process of sinking P to start over again.

I've never heard of a calcium reactor been required because the system has become eutrophic. They are usually used in oligotrophic systems with great SPS growth.

this is to keep the alk levels up more than the Ca levels. alk is a big source of elemental C for bacteria. the more bacteria, the more elemental C. vodka dosing is just an even easier source of C for bacteria.

with 7 clams (2 over 5") and plenty of colonies and frags of SPS, i did not have a problem keeping my alk, Ca levels at NSW levels with just kalk in the top of water.

there is an amazing mount of resources used in this hobby to cover up the affects of P and N in our systems, because we have been told to keep poo as a pet for so long.

G~

 

[simonh]

you do have the deep sea, it is the substrate. in our systems that is the P sink.

I'm never going to buy the argument that a 4-6" sand bed is equivalent to the deep sea.

if it does not contain N and P, then what does it contain? anything that can be used as food for another organism must contain N and P.

There are lots of solids that needn't contain N and P. After it's passed through enough organisms and been broken down by bacteria to extract N/P there may be no nutrition left. For the sake of this argument we don't have to worry about what's in it as your argument is all based around P sinks.

correct, you can ignore it until the sink becomes full, then all of the P that was going into it can not and all of the P now stays in the water column. just like you never have to worry about changing the oil filter on your car.

correct. there is a gradient, and that is why the substrate acts like a buffer up until it gets full. the finer the substrate the greater the gradient can be. the more coarse the less gradient and the better the equilibrium can be maintained. this of course means that affects of the sink filling up will be seen faster. as is seen by people who use coarse substrates instead of fine substrates that follow the same maintenance routines.

Again you talk about things becoming full... but in the next sentence about equilibrium. As though some switch becomes flipped at some point and the reaction changes from storage to releasing...

do you really want the P to be in equilibrium with the water? by the time this occurs you are already behind the curve. we would rather have the higher P in the water and the lower P in the substrate at the boundary layer. luckily the bacteria make this happen for us. the best way to go after N and P is by removing the detritus before it can become a nutrient source for bacteria. it makes more sense to me anyway to use the substrate P sink at what it does best, bind P. you remove the detritus from the substrate on a regular basis (based on the trophic level wanting to be emulated, this could be weekly or after several years, or nearly never) allowing the process of sinking P to start over again.

But often we are behind the curve. A few weeks/months skipped maintenance etc. I'd rather have a sand bed with some fauna helping out whilst I'm busy rather than it accumulating on the bottom of the tank and causing hair algae etc. or stopping the corals from calcifying. With a sand bed when I return to maintenance clean the skimmer, get the PO4 levels down the sand will come into equilibrium with the water column, my skimmer will export more food I put in and less end up in the sand and the sand bed levels in fauna and P will return more to normal levels.

this is to keep the alk levels up more than the Ca levels. alk is a big source of elemental C for bacteria. the more bacteria, the more elemental C. vodka dosing is just an even easier source of C for bacteria.

I'm not sure the relevance of the alkalinity and elemental C and the relationship to vodka dosing.

with 7 clams (2 over 5") and plenty of colonies and frags of SPS, i did not have a problem keeping my alk, Ca levels at NSW levels with just kalk in the top of water.

I'm not sure how much evaporation you had. But given the amount of calcium in saturated limewater and quantities you could calculate the calcification rate and compare it to a natural reef. I for one have experienced coral growth that was able to way outstrip the amount limewater can theoretically supply.

 

[Reefin' Dude]

I'm never going to buy the argument that a 4-6" sand bed is equivalent to the deep sea.

that is fine, then think of them as a lagoon substrate. the problem with that is that we now have to explain why we are not doing regular major substrate disturbing events. those pesky tropical storms. we are emulating nature right, or are we?

There are lots of solids that needn't contain N and P. After it's passed through enough organisms and been broken down by bacteria to extract N/P there may be no nutrition left. For the sake of this argument we don't have to worry about what's in it as your argument is all based around P sinks.

ok, but still this is an increase in material. an increase in material is an increase in mass of the system. a sign of eutrophication. i guess this extra material could be trace elements, but that would lead to a toxic sludge, also something not wanted in our systems. any way you look at it, this material is not something we want in our systems indefinitely.

Again you talk about things becoming full... but in the next sentence about equilibrium. As though some switch becomes flipped at some point and the reaction changes from storage to releasing...

when do you decide to take the trash out or flush the toilet? when does the trash can stop being useful for keeping trash? you can keep putting stuff in it when it is full, but it just falls to the floor. i think you are missing the role bacteria have in this and the sand itself. take the very top layer of a substrate. it is completely clean, and so is the water. now lets a touch of P to the water. to reach equilibrium the sand binds the P to reach equilibrium. now comes the bacteria. they take the P off of the sand, they fall into the substrate. now there is an open site for the P if the water and the sand are not in equilibrium. this poor bacteria with the P either lives, dies, or gets eaten. either way the P is now organically bound. the P is still sunk in the substrate and has not changed the equilibrium with the water column. this just keeps going on and on. the bacteria take the P, then do whatever. leaving a new place for P to be bound from the water column. of course some of this organically bound P will become inorganically bound by bacterial action, but as they are deeper in the substrate the harder it is for the P to reach an equilibrium with the water column because of all of the material in the way creating a barrier.

But often we are behind the curve. A few weeks/months skipped maintenance etc. I'd rather have a sand bed with some fauna helping out whilst I'm busy rather than it accumulating on the bottom of the tank and causing hair algae etc. or stopping the corals from calcifying. With a sand bed when I return to maintenance clean the skimmer, get the PO4 levels down the sand will come into equilibrium with the water column, my skimmer will export more food I put in and less end up in the sand and the sand bed levels in fauna and P will return more to normal levels.

there is nothing wrong with that, and a substrate can be used to great affect in this manner for a while. a skimmer is not going to get it all, and in fact if the calcium carbonate bacterial tug of war did not exist our systems would not be alive. skimmers are fantastic exporters of organic P. i think they need to be bigger though. i am also a big fan of settling tanks, think empty live sumps. a place to see and easily remove accumulating detritus. conical setting tanks being better because of the nice valve on the bottom.

there is always going to be an increase in P in an undisturbed substrate even with the biggest skimmers or the most flow. it is the calcium carbonates ability to bind with inorganic P that makes the whole shebang seem to work. those that use substrates are relying on this interaction. it is the substrate that is keeping the N and P levels the way they are as long as they are without significant disruption of the substrate to clean it out.

the need for GFO and like are all indicators that buffering ability of the substrate is running out. you are relying on the GFO (or whatever the P binder du jour is) as the new P sink instead of the substrate.

I'm not sure the relevance of the alkalinity and elemental C and the relationship to vodka dosing.

this can get deep in a hurry if we need to down this rabbit hole. to sum it up. all organisms need a food source and building material. to simplify, lets say N, O, C, and P. they need all of these. we talk about P and N, but in fact the same bacteria that work on the N compounds are also up taking P, C, and O also. depending on what bacteria are doing the work, they may find one of those elements limiting (lacking). in our systems, this is most likely C. we use vodka, and other sources of carbon to feed the bacteria in order for them to do their magic on P and N. we then skim off these bacteria as a way to export the N and P that they contain. CO2 and carbonate are just other sources of elemental C for bacteria.

I'm not sure how much evaporation you had. But given the amount of calcium in saturated limewater and quantities you could calculate the calcification rate and compare it to a natural reef. I for one have experienced coral growth that was able to way outstrip the amount limewater can theoretically supply.

i would bet it is not your hermatypic organisms, but the bacteria that are stripping the water. Bomber also only used Kalk for his alk, Ca requirements. the more waste, the more bacteria, the more C needed, the less C available for the corals (the organisms that you actually care about), the more C you need to add to keep everyone happy.

G~