How Freshwater Deep Sand Beds Work

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Seeker of Piscean Wisdom
Original poster
Apr 10, 2009
San Francisco
Ok, the thread I started on California Blackworms got me thinking about the sand bed in my tank. It's three inches deep, and has planaria, cali blackworms, and MTS for critters and it's a planted tank with plenty of roots, extending usually 1.5 to 2 inches in depth.

My tank water never goes above 10ppm NO3 even though I almost never do water changes. Also, the NH3/NH4 and NO2 levels are consistently zero. All this despite the woefully inadequate filter I chose to replace the B9's stock filter with.

So, I believe deep sand beds can and do work in freshwater aquaria. I thought, since I'm suddenly in a slump for inspiration for the blackworms thread, why not put something up to do with FWDSB? (Freshwater Deep Sand Beds)

My experience with them is that: They are not hazardous to fish and invert life, they control nitrates, mulm disintegrates to the point where it sifts into the sand to provide yum yums for blackworms and rooted aquatic plants, that even plants requiring rich planting media will thrive once the sand/mulm layer is established, they do not require stirring or vacuuming, that cories and kuhli loaches love DSBs, and that my water is consistently of good quality.

I know not everyone's experience agrees with mine and I believe that is because of the differences between our particular FWDSBs. My theory is that my DSB works the way it does is because of the planaria, cali blackworms, mts, rooted plants, and because the sand is coarse with rounded grains and doesn't pack down very much at all.

I'd love to see a discussion about the different variables that can go into a FWDSB and what works how and why (or why not).

Though my tank is not a Walstad tank, it is inspired and informed by her method and information as presented in "Ecology of the Planted Aquarium." I suggest that book as a great source of information about nutrients, bacteria, lighting, etc., comprehensible to the layperson

Forgive me for trying to start another big information thread but the topics just seem so interesting!


you are hypnotized! LOL
May 31, 2005
fairchild wi 54741
Real Name
douglas harvey
the deepest sand bed for my tank is 2.5 inches and the shallest is aboug 1 inch maybe 1.5 and i only stirred the sand one time since i got it and its been a month almost so far so good


AC Members
May 29, 2009
I was thinking about this a while back. I dont think it would be possible for that to sustain itsself over long periods of time if it was stocked with a bunch of fish that create alot of waste. Even if you had blackworms that feed off of the bacteria and/or mulm in the substrate, it would be hard to mantain that "colony" of worms because theyre food for most fish in your average aquarium.

What size is your tank? It seems like this would be ideal for a small invert tank with inhabitants that dont create an abundance of waste(my Brigs are EXTREMELY messy). Have you thought about trying a Walstad tank? Ill bet the benefits from the blackworms would increase exponentially in a setup like that!


Certified Dubhead
Dec 14, 2009
Real Name
yeah i have about 2-2.5 inches of sand in my tank and a still have about 20% of sand left in the 50lb bag. Should i add the rest? because i always wanted to have nice deep sand to make hills and whatnot and to make it a little more intersting.


Jun 13, 2009
In the Black worms thread dee shows how the blackworms are quite quick and apt at avoiding hungry fish, but they still need to be replaced due to fish eating them, but this is solved my the aquisition of more blackworms.


Seeker of Piscean Wisdom
Original poster
Apr 10, 2009
San Francisco
I was thinking about this a while back. I dont think it would be possible for that to sustain itsself over long periods of time if it was stocked with a bunch of fish that create alot of waste. Even if you had blackworms that feed off of the bacteria and/or mulm in the substrate, it would be hard to mantain that "colony" of worms because theyre food for most fish in your average aquarium.

What size is your tank? It seems like this would be ideal for a small invert tank with inhabitants that dont create an abundance of waste(my Brigs are EXTREMELY messy). Have you thought about trying a Walstad tank? Ill bet the benefits from the blackworms would increase exponentially in a setup like that!
Well, my tank is an Odyssea B9, nine gallon bowfront nano tank, with an 18w CF so it's nice and bright. In this photo, I'd just planted some hairgrass and kicked up a lot of white haze from the sand (the haze is disintegrated, decayed mulm from in the sand).

I dismantled the built-in filter, which was made for keeping a nano-reef, replacing it with an internal one which had come with an Odyssea Cube-5 (five gallons) and which I threw out the filter cartridges from and then stuffed with poly-fill and topped with some sintered glass bio media.

Actually, I am embarrassed at how I've stocked it. An example of poor impulse control, it is. There are otos, corydoras harbrosas, glowlight danios, flame rasboras, endler's livebearers, a blue ram, and kuhli loaches. The total inches? I take the fifth. They're all anywhere from very small to tiny, so their bioload doesn't add up to beyond what the tank can handle, just barely. Anyhow, let's just say it's overstocked and the fishes do create an abundance of waste! And they do eat a lot of worms but only a little faster than the worms reproduce so every month or two I go to the LFS and buy a buck's worth more of the worms and re-seed the tank; takes five minutes with a turkey baster. Over the last couple of years, I've wondered where all the waste is going. Surely the worms aren't eating it all but then the only explanation I can think of is that it continues to sink deeper into the sand and decay and break down into smaller and smaller particles and eventually most of it becomes molecules of various chemicals as the bacteria metabolize it. I suppose after enough years go by the indigestible components of the mulm will have to clog up and the interstices in the sand and at that point I'd need to break down the tank and redo the substrate - though that would have to be a long time from now.

I'm sure blackworms would thrive and be very beneficial in a Walstad tank. I'm interested, however, in working with deep sand beds in freshwater tanks. There are a few reasons, such as the ease of pulling up and planting plants without releasing clouds of mud, the ease of observing the goings-on in the sand next to the glass (soil is too dark for me) such as anaerobic and anoxic areas which turn first grey and then black as O2 levels drop, and the fact that I can just set up a tank with a sand bed and use a turkey baster to suck up some sand from an established tank and squirt it into the new tank plus add some blackworms and planaria and mts and some plants and there I have the beginnings of a live deep sand bed which will propagate itself (except for the blackworms which usually need reseeding every few weeks/months) with just a few minutes work. Also, I just like sand a lot and I hate gravel, which is what Walstad tops her soil with (she writes she's had poor results using sand to cap the soil). However, I've thought a lot about mixing 10% soil by volume with the sand and using just a very thin layer of plain sand to cap the soil in. However, the problem with plumes of silt coming up whenever planting or pulling plants would still be there, though hopefully less since there'll be 90% less soil.

I've no idea how quickly it would cycle but I bet seeding, say, a 20gal with about a cup of freshwater live sand thrown in on top of its fresh sand bed would get it cycled quite quickly. I'll have to take notes when I start my next tank this way - which will be very soon! Either another Odyssea model (Cube 20)(what can I say, I'm poor) or MAYBE the JBJ Nanocube Deluxe 24gal I saw on clearance at the LFS I usually NEVER visit because of their crappy livestock. My problem with the nanocube is the built in filter-too powerful and not adjustable enough. I'd have to jerry-rig something to decrease the filter volume. My problem with the Cube 20 is its lighting, which is just a tad too dim for me at one 36w CF - also the lighting is much stronger directly under the CF than to either side. The nano has 2x36w CF, more evenly distributed and the Cube has no pesky built-in filter so I can make my own choice in "underpowered" filters. Also the Cube is like $60.00 only.

Anyhow, my idea is that one can start with a tank and plain sand and water, and then add a kit composed of a bunch of live sand, blackworms, MTS, planaria, and some hardy plant which propagates quickly by runners perhaps. Then you'd just have to wait and add a little fish food to feed the snails and also the bacteria the worms and planaria eat. Measure the NH3/4 and NO2 levels while the plants and critters multiply, until it's cycled and voilá! You have a FW DSB tank! Just add fish. I guess somebody could actually sell such a kit mail-order next day air.
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Seeker of Piscean Wisdom
Original poster
Apr 10, 2009
San Francisco
yeah i have about 2-2.5 inches of sand in my tank and a still have about 20% of sand left in the 50lb bag. Should i add the rest? because i always wanted to have nice deep sand to make hills and whatnot and to make it a little more intersting.
Sure, I'd add a little more. My favorite LFS's tanks are ALL FWDSB (though not quite the same technique as mine) and heavily planted and there is always more than two inches and often four or five inches of sand, sometimes landscaped into hills and whatnot.

If the tank is already established and cycled, DON'T smother the current top layer of sand. Pour your (prewashed so it doesn't cloud everything up) sand on top and then mix it up well, stirring the old top layer up into the new top layer so the at least some of the existing nitrifying bacteria in the sand will be brought to the surface. Otherwise, you may be in for another cycle or mini-cycle.

Sand is a wonderful medium for biological filtering. The top 1/2" to 1" of a good, fluffy, loose-packing sand (like a coarse, quartz-based river sand) will be aerobic and has scads of surface area, serious surface area. It will host nitrifying bacteria(get rid of ammonia and nitrite), and below that, denitrifying bacteria(get rid of nitrate). Simple diffusion gets the O2, NH3/4, NO2, and NO3 to and from the surface and the layers below rapidly enough that a tank with no other filter that the FWDSB's biological filtering can maintain a surprisingly dense population of fishes. Of course, with MTS, blackworms, and rooted plants opening up the sand and moving it around the DSB can handle even more. And denser, closer packing sand has lesser capacity - the finer the sand, the denser it can pack.

Conversely, if the balance of your tank includes a FWDSB and you smother it, the nitrifying bacteria will have to multiply in your power filtration system until there's enough to pick up the slack. In the mean time, you have a cycle.


Seeker of Piscean Wisdom
Original poster
Apr 10, 2009
San Francisco
Who does what in my deep sand bed?

First of all, there is sand. Sand is essential to have a sand bed:D 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


Malaysian trumpet snails

In addition, some other organisms contribute but I don't think they're needed:

Tiger shrimp

Kuhli loaches


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.


old school newbie
Aug 2, 2008
westminster sc.
gotta agree with slappy here. i've enjoyed your posts a lot