Who does what in my deep sand bed?
First of all, there is sand. Sand is essential to have a sand bed
Also, the nature of the sand you use will greatly effect how your sand bed performs. I recommend the kind of sand I use, which is a quartz based, fairly smooth grained (as opposed so sharp grained sand - which will hurt your cories btw) sand. What I get is a river sand that the guy at my LFS pre-washes and repackages so I haven't figured where to procure the stuff on the cheap.
This kind of sand doesn't get very packed down and the top layer could be described as almost fluffy. "Fluffy" sand is good for a few reasons. One is that water and dissolved gasses and nutrients can diffuse through the sand more quickly and for greater distances than in packed sand. Another reason is that mulm can more readily sift down into the sand when it disintegrates. Also, worms and snails and other critters can burrow into it more easily.
It's important that things can diffuse through the sand since that's how the ammonia, nitrite, and nitrate can reach the various bacteria which process them and keep the water quality high. Also, it's how nutrients move through the sand and reach the rooted plants.
The burrowing is important because it enhances the movement of nutrients, oxygen, and nitrogen compounds through the sand.
Finally, it is essential that mulm be able to penetrate deep into the FWDSB. Practically everything else can go but for bacteria, sand, and mulm and the FWDSB would still function at least marginally. Take the mulm out of the sand and you'll diminish the function to the point where it is quite unremarkable.
There are four primary organisms aside from the protozoans, bacteria, and microfungi which live on/in the sand:
Rooted plants
California blackworms
Planaria
Malaysian trumpet snails
In addition, some other organisms contribute but I don't think they're needed:
Tiger shrimp
Kuhli loaches
Scuds
Pond snails
And last but not least, an essential non-organism: Mulm
dead stuff
Further, there may be any number more of useful critters but I just don't know of them. If you do, PLEASE post about them!
What they do (or I think they do at any rate)
Rooted Plants: Plant roots not only absorb nutrients from the soil, but they actually can carry oxygen down into the soil to create a microenvironment right around themselves which they can survive in when they penetrate severely anaerobic and anoxic regions. This oxygen facilitates the action of aerobic bacteria much deeper into the substrate than it otherwise could reach.
Some aerobic bacteria which live down amongst the roots oxidize hydrogen sulfide while others metabolize methane, alcohol, and other metabolic byproducts of anoxic and anaerobic bacteria. This way, plants greatly contribute to the safety of deep sand bed regions where toxic gases and chemicals are formed.
Also, plant roots open up the sand as they grow and help keep it from compacting.
Also also, when the plant dies or if it is cut off at the roots, the root system dies and decays, releasing CO2, minerals, NH3/4, and other nutrients back into the soil. Decay is an important source of CO2 in many aquatic ecosystems and it assists plant growth. Ammonia/ammonium is absorbed by nitrifying bacteria to create nitrates and also by plants as their favored source of nitrogen. They decaying roots disintegrate and loosen the sand once again.
Also also also, plants absorb heavy metals and other toxins from the substrate before they can leach into the water.
California Blackworms: What can I say? They're tops! One super thing blackworms do is burrow from the surface to the start of the anaerobic layer, opening up the substrate and assisting the circulation of and diffusion of water and gasses up and down that first approximate .5 to 1 inch. This helps get the anaerobic layer where nitrate is reduced to N2 or N2O and is released as bubbles.
Also, the worms eat decaying organic matter and bacteria, transforming some of it by metabolism into energy and soluble waste molecules like CO2 and NH3, which is great for the plants, and breaking the rest of it down into finer and finer particles which are excreted as waste.
Also also, the worms attract loaches and cories as well a some other fish, which go after them so enthusiastically that they dig up the sand. Especially the kuhli loaches, which practically are burrowing - to a depth of about .5 to 1 inch - at times to reach the worms. And, basically, worms are great food for the fishies.
Planaria: What are planaria? They are flatworms, segmentless, flat, worms. They are small, usually .25 inches and less. They eat fine organic matter and bacteria, and will crawl in between grains of sand to do it. Oh, and they a closely related to flukes, which are parasitic flatworms.
Essentially, the planaria go more places in the upper substrate than do the blackworms, which basically stick to their favorite burrow. They barely, if at all, disturb the sand when they crawl into it so they're no help with oxygenation and circulation. What they are good at is taking very find organic matter and converting it into a. more planaria, b. metabolic products like CO2 and NH3, and c. incredibly fine organic matter, which is good because the finer the matter is the faster bacteria will finish breaking it down into metabolic byproducts.
So planaria are a very good though narrowly specialized member of the cleanup crew.
Malaysian Trumpet Snails: The good old MTS. If you didn't already know, these guys like to burrow in search of whatever it is they eat. They really do a good job of turning over the sand. So much so that they are seen by some as the solution to the problem of toxic, anoxic pockets in the sand which will eventually release H2S and whatever else and kill all the fish in thirty seconds. The snails are supposed to burrow 'most everywhere and mix up the anoxic with the anaerobic and aerobic sands.
Personally, I don't buy it. I've never seen any sign that my MTS are burrowing deeply enough to mix it up that way. I'd be seeing black sand sometimes appearing on the surface and I see them burrow sometimes next to the glass and they never go even as far as 1 inch down. If anything is protecting my particular FWDSB tank from H2S it is probably the rooted plants (see above)
What these snails do is turn over that important top layer and help exchange gasses/chemicals up and down. They also seem to eat organic matter and microorganisms under the sand so they probably help in a similar way to planaria.
Tiger Shrimp: NONESSENTIAL, I believe, though I happen to have 'em. These guys mess around on top of the sand, eating random crud and breaking it down. They also pick up grain of sand after grain of sand, helping small particles of mulm get a start on sifting into the substrate. These li'l guys are nonstop action and I'm sure they get a lot of mulm to start on it's path to disintegration and into the sand.
Kuhli Loaches: NONESSENTIAL, I believe, though I have 'em and love 'em. These little guys like to dig down into the sand for anything or any reason which pleases them. Maybe for fun, who knows? What I do know is that they love eating blackworms and dig down after them over and over, usually unsuccessfully. So, they do the same thing that MTS do, just faster and less methodically. I suppose if you didn't want MTS and you wanted kuhli loaches , so long as you had blackworms in the sand, the MTS wouldn't be missed.
Scuds: NONESSENTIAL, I am sure. Otherwise known as amphipods, these guys look and act like if you crossed a shrimp with a cockroach. They're quite small and from a distance look just like some random, light colored bug. Amphipods eat small bits of crud and poop out much smaller bits. Basically like shrimp but smaller. They can help break down mulm.
Pond Snails: NONESSENTIAL, I'd bet on it. Know as bladder snails, also. They crawl around eating anything tasty, leaf litter, biofilm, old fish food, and god only knows what else. They help break down mulm and protomulm.
What I use pond snails for is this: I catch them and crush them in my planting tongs. Then I stick the squished snail and its shell a couple of inches into the sand, not far from some lucky plant. This way, the snail has gathered up a bunch of trash and without knowing it, become fertilizer.
Dead Stuff: Kind of NONESSENTIAL, but not precisely. Mulm is not living stuff, and may have dead stuff in it like leaf litter. Anyhow, if a small fish dies, say like 2.5 inches and under, you can do like I've done and bury it way at the bottom of the sand. Also, you can leave severed but intact root systems in the sand or even bury some plant material here and there, but not too much. This stuff will all decay and create anoxic or anaerobic zones around itself, which is why I say bury it at the very bottom of the sand, three inches down. It will release nutrients into the sand, and gases as well. Be sure only to bury small amounts (like a pond snail) at a time or at least have plants planted very close so their roots will invade the zone of decay and disarm the toxins with their oxygen microenvironment.
I've never had any problems at all from this practice but I urge caution. If you are too liberal with burying stuff which will decay, you will surely end up with pockets of toxins which overwhelm the capacity of the aerobic layer and rooted plants to neutralize. In other words, you can kill everything or at least foul your tank pretty easily. If you feel unsure, probably you shouldn't do this crazy thing and if you're feeling very sure of yourself, you probably definitely shouldn't be doing this crazy thing.
So I describe a practice and then say don't do it? So what? I've never been all that consistent anyways.
Mulm: ABSOLUTELY ESSENTIAL. Mulm is the source of energy and/or nutrients for the FWDSB. The bacteria which perform all the important chemical reactions, for converting ammonia to nitrates, nitrates to nitrogen, releasing locked-up mineral nutrients, neutralizing hydrogen sulfide, etc. either just get some essential mineral nutrients from the mulm, like autotrophic bacteria that burn NH3/NH4 or H2S for energy do, or they get those nutrients plus they aerobically or anaerobically metabolize the organic compounds in the mulm for energy, as heterotrophs do.
Mulm provides food for the blackworms, which burrow in order to find their favorite mulm eating bacteria. It also provides mineral nutrients which bacteria unlock so they become available to the root feeding plants in the sand.
When it is getting started up, a FWDSB of the type I describe takes a while to reach its potential since at first there's no mulm to sift down into it. It has occurred to me that a thin layer of 10% soil/ 90% sand laid down, capped by .25 to .5 inches of plain sand, all on top of the remaining sand bed would kick start the whole process. Or perhaps one could compost some fish food in water, allowing it to decay for a good while and then inject it with a turkey baster directly into the sand here and there.
At any rate, decaying organic matter in the sand is essential. Without it you could have some autotrophic bacteria like nitrifying, which would have to depend on the mineral content of the aquarium water to get those elements which are essential to their life. Other than that, no much would be goin' on. Root feeding plants would be at best hum-drum and would need root tabs.
To have a rich, mulmy, sand bed I simply don't vacuum. I don't overfeed and I don't vacuum. If I had a more modestly stocked aquarium I would probably slightly overfeed á la Walstad but I've got so **** many fish their poop is plenty.
ALRIGHT ALRIGHT, I've stated all this authoritatively and as facts. I must qualify this entire post with the statement that I don't really know what I'm talking about and frequently confuse my own hypotheses with real facts.
What I mean is, this whole posting is like my theory of how my particular FWDSB works and the principles by which others should be able to work. I use a bunch of facts but a lot is either just from trial, experience, and observation or my theories about what I've observed, and my best guesses.
Again, please forgive my wordy posting. I just can't help myself; I love being didactic. It helps me organize my own thoughts.
