Hydrogen Sulfide gas is a gas produced in an-aerobic pockets under water. These may occur anywhere there is deep sand or sediment, or in a sump with thick mulm.
Ponds with more than a few inches of gravel on the bottom will develop noxious gases including methane, Hydrogen sulfide and toxic levels of Carbon Dioxide.
Without Oxygen, some bacteria can respire by using Sulfur, and this creates Hydrogen Sulfide, H2S that is very, very toxic to fish. HACH Chemical has a test for this, (1-800-227-4224) but this should be suspected in any case of fish loss where:
- There are no parasites, proven by microscopy,
- There are no real Ammonia or Nitrite derangements
- The signs are respiratory and
- The history has some reference to a stalled filter or the stirring of stagnant gravel or media.
The smell is one of rotting eggs. Losses may be great and they will continue after the H2S is long gone.