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 can exist in an aquarium in the substrate if the substrate is deep enough. Per these individuals, conditions in some substrates (generally something called a “deep sand bed”) are anaerobic or anoxic and reduce nitrate to nitrogen gas and reduce sulfates to hydrogen sulfide.
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 sugarcoat it.
Deep sand beds do NOT do ANY anoxic denitrification nor do they create hydrogen sulfide.
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, three months, three years, etc., etc. ALL combinations will NOT produce anoxic denitrifying conditions or any hydrogen sulfide gas. It just CANNOT happen for no less than six very solid scientific reasons.
Year-long testing of four 5-inch deep sand beds with two controls showed they did NO anoxic denitrification nor did they create any hydrogen sulfide gas.
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):
14.2.4.1. The Science behind the Myth of the Anaerobic Aquarium Substrate
For more on the anaerobic myth click on the following:
7.5. Denitrifying Media
8.9. Anaerobic Reactors
Startpage Aquariumscience
Source: Aquariumscience.org – David Bogert
