Hydrogen sulfide (hydrogen sulphide in British English) H2S, is a colourless, toxic, flammable gas that is responsible for the foul odour of rotten eggs. It often results when bacteria break down organic matter in the absence of oxygen, such as in swamps, and sewers (alongside the process of anaerobic digestion). It also occurs in volcanic gases, natural gas and some well waters.
It is harmless if it naturally releases itself in small amounts from the gravel and this can be seen as small bubbles appearing from no where from the substrate.
But if the substrate is disturbed during maintenance or an animal burrowing then it can release large amounts which dissolve rapidly in to the water and kills.
- It is heavier than air, so it can accumulate at the water surface if there is not adequate ventilation in the tank hood and so cuts off oxygen from entering the water and so kills the animals that way. Look out for gasping fish.
- It has been often reported of aquarists moving a large piece of ornament after a long time and having large quantities of this trapped gas given off and cause fish to die fairly quickly if exposed to it.
To reduce the levels of this gas, the substrate needs to be kept stirred with a long stick once a week. Or the layer of substrate is kept relatively thin (around 1" thick). Adding plants to the substrate will reduce this gas as their roots aerate the gravel.
Or add your own waste control bacteria that out-competes the natural H2S producing bacteria so preventing this situation in the first place.
Sand substrates are the worse (or the best depending on your point of view) for promoting this gassy bacteria.