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.
Hydrogen Sulfide Pond Gas – Not Very Common
Without Oxygen, some bacteria can respire by using Sufur, and this creates Hydrogen Sulfide, H2S which is very, very toxic to fish.
HACH Chemical has a test for this,
P.O. Box 389
…but H2S intoxication should be suspected in any case of fish loss where:
1) There are no parasites, proven by microscopy,
2) There are no real Ammonia or Nitrite derangements
3) The signs are respiratory and
4)The history has some reference to a stalled filter or the stirring of stagnant gravel or media.
5) NEUROLOGICAL signs, of spinning, flipping and fearlessness may be seen just before death occurs.
The smell is one of rotting eggs. Losses may be great and they will continue after the H2S is long gone. If you discover or suspect H2S poisoning, you should REMOVE THE FISH before disturbing ANY gravel or sand.
Also, to neutralize SOME of the hydrogen sulfide, a weak dose of Potassium permanganate may be effective. Still, housekeeping, and finding the source of the anaerobic bacteria is the priority.
I’ve heard from a LOT of people that Aquascape ponds have Hydrogen Sulfide problems. I have tested dozens of these, especially the older ones since they are the most lored to have a problem – and have found NO H2S. My kit is the pad test from Hach. H2S is not an issue of pond TYPE but purely of pond maintenance.