One of the biggest myths, repeated over and over by a host of well meaning but ill-informed commentators on social media, is that anaerobic or anoxic conditions exist in a normal aquarium in the substrate. Per these individuals, conditions in some substrates (generally something called a “deep sand bed”) are anaerobic or anoxic and reduce nitrate to nitrogen gas. Now I know many will “waffle” on this topic as they do not want to offend all these commentators. I cannot do that. All these commentators are just plain wrong! Sorry, can’t sugar coat it.
This is true no matter how you set up the deep sand bed. Small sand particle, large sand particle, kitty litter, laterite, gravel, no plenum, plenum, very slow flow, no flow, slow flow, moderate flow, organic matter, no organic matter, etc. etc.. ALL combinations will NOT produce anoxic denitrifying conditions. It just CANNOT happen for no less than six very solid scientific reasons.
Year long testing of four 5 inch (12.5cm) deep sand beds showed they did NO anoxic denitrification.
Another myth is that there are anaerobic or anoxic substrates which produce poisonous hydrogen sulfide gas bubbles. This is also patently false. Unequivocally:
This is NOT to say that LOW oxygen conditions cannot exist in the aquarium. Low oxygen (Hypoxic) conditions can occur in an aquarium if a filter is turned off or an ornament has a deep pocket where uneaten food can accumulate well away from any water flow. If an aquarium is poorly aerated, overstocked and/or overfed this can be acerbated. Under these conditions organic bacterial toxins along with ammonia and nitrite (and some very bad smells) can be produced which can kill the fish. But it needs to be emphasized this is VERY RARE.
Note that the distinction between anoxic and anaerobic is very blurred, with the two terms being used interchangeably in all the literature. We go into the exact meanings in the article linked below on the science but for now assume anaerobic and anoxic mean the same thing.
This anaerobic myth is most commonly associated with something called a “deep sand bed”, where a bed of sand, especially a bed of sand in a marine aquarium, supposedly becomes anoxic and supposedly does a lot of nitrate reduction to nitrogen gas. There is a whole lot of “science” in marine aquarium keeper websites on how “well” this works.
Indeed there is a whole community of hobbyists whose passion is “deep sand beds” or “DSBs”. These hobbyists trade anecdotal evidence about how well DSBs reduce nitrates. This is all simply misinterpretations of what they are seeing. Invariably the real reason for the reduction in nitrates is “assimilatory denitrification” where plants, bacteria, fungi or fish are incorporating the nitrate into their bodies.
But it is also common for well meaning but ill-informed commentators on social media to say that normal substrates such as sand and gravel have “gone anoxic” and produced hydrogen sulfide gas. They then tell folks this hydrogen sulfide gas killed their fish. This is simply patently impossible in any substrate in an aquarium. Period.
The Anaerobic Science in More Depth
I go into the science behind the anaerobic substrate myth in a great deal more depth in this article (note I recommend this boring lengthy article only for the nerds among us, like myself):
For more on the anaerobic myth click on the following: