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Substrate depth?

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I'm curious as to how deep you can safely have your substrate.  I understand that some shrimp keepers have bare bottom tanks.  Mine is about 2 inches deep (I think it is active-flora?  Some king of "rock" type).  The tank has fish as well as shrimp, and is heavily planted.  I have been under the impression that a deep substrate can trap bad gases, which can all of a sudden burst out & kill the tank inhabitants.  But I've also seen tanks that are very generously sculpted with substrate.  Things like very deep in the back, shading down to a shallow front.  Or big hills and shallow sides... it's kind of confusing.  Does anyone have an opinion?  Educate me, please!  :D

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Usually it's said that 3" is the max depth before you start having anaerobic areas in the substrate. It does depend on the particular substrate though. If it compacts easily, then less oxygen/flow would reach deep into the substrate, so with a substrate that compacts easily/tightly, you may only be able to go 2" or so.

Pool Filter Sand has very uniform size grains so it doesn't compact itself as much, so you could go a bit deeper (but 3 inches is usually enough for planting)


On the topic of the dangerous anaerobic pockets creating potentially harmful gasses and bacteria/microorganisms, that is up for a huge debate. I, myself, have never gotten to the bottom of the discussion. It's said that the Hydrogen Sulfide gas pockets/bubbles created in anaerobic substrates, once it hits oxygenated water, it instantaneously oxidizes and is rendered non-toxic in an instant, so it is safe.

If that is true, and if Deep Sand Beds (anaerobic substrate) being able to grow anaerobic denitrifying bacteria that would convert nitrates into nitrogen gas, were true, then that would be a bonus.

But I am not completely convinced that is all true. I have seen old tanks that had their substrate disturbed (not regularly vacuumed) and fish would die soon after (some cases have reported instant deaths).

Some say it is "ammonia pockets" in the substrate. It makes sense that kicking up mulm/detritus/organic matter from under the compacted substrate would provided a lot of food/energy for Heterotrophic bacteria to decompose/breakdown that organic matter into ammonia, which would be deadly, but I have heard and seen instant fish deaths right after  (less than 2 minutes later) stirring up the substrate, so I kind of doubt it was ammonia spikes (happened too quickly in my opinion), it seemed the quick deaths may be a result of the gases that were released. So I can't say I fully agree with those gases being said to be non-toxic (maybe if a fish gets directly in a rising gas bubble or intakes it, then the gas is still toxic? ie the gas in the bubble is toxic, but any "dissolved" gas in the water column is rendered non-toxic). The gases are harmful to humans, so don't inhale so much of the released gas at once (Hydrogen Sulfide smells like rotten eggs)


As I said, it's a debatable topic. I've been meaning to try and find a forum with some scientists that may be able to get to the bottom of it/true facts, but so far I haven't come across a forum to dive deeper into it.


Either way, the gases being harmless or not, anaerobic substrates mean they have very little oxygen, and plant roots need oxygen. And I can imagine the anaerobic substrate creating conditions for anaerobic bactera (talking about diseases), which would not be what I would want. For now, until I learn more about it, I just play it safe and try to minimize having an anaerobic substrate.


Those aquascapes with large hills, often use material (like egg crate, rocks or actually creating formations with DIY fiberglass, etc) under the substrate to create the slopes and give it stability.

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My debris tank has 1.25" of compost with 1"of pebbles on top.  Animals doing well , but the rooted plants not doing as good as in the inert substrate tanks.  ShrimpP has it right I believe in that not enough o2 in the compost due to compaction.  Nutrients are locked up in the compost so they are not available to the plants.  Floaters doing fine due to no3 from fish. Still love my natural "ditch tank".

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So does this mean that if the substrate is full of plants and roots then it should be okay? My office experimental tank has a sand substrate with lots of plants and the foreground is dwarf hairgrass which is spreading across the only "bare" area. To vacuum in the sand is to disturb plants and I haven't figured out how to vacuum the sand deeply with a hand vac. I'm hoping the plants keep things stable. The substrate is less than 2" but the sand is fine and packs easily. Opinions?

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Surprisingly I haven't kept any MTS yet, but I have heard they only dig within the couple or so inches, so if that's true, it wouldn't be of much benefit. Can anyone confirm whether or not Malaysian Trumpet Snails do or do not go deeper than 3 inches into the substrate?

California Blackworms and perhaps detritus worms would aerate the substrate more at deeper depths (not sure how deep they go though).


I wouldn't be worried about nutrients being "locked" below a compacted sub or a compact sub keeping nutrients in the water column from getting to the plant roots, as the plants can uptake nutrients from the sub with their roots, and all plants do just fine only being fed nutrients through the water column via their leaves. Just more concerned about the anaerobic substrate having a lack of oxygen (roots need oxygen), and the anaerobic sub does create an environment for anaerobic bacteria that would take up residence on the plant's roots (hence why you may see black roots, some roots die, some just get covered in "black" from the bacteria/gases).

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