I have always had doubts as to the prevalent belief that fish will automatically die in a completely iced over pond. Naturally occurring ponds in the Northern latitudes are prone to completely ice over every winter, yet the fish remain alive. If they didn't, these same ponds would be completely devoid of any fish and we know that is not the case.
Hydrogen sulphide gas is formed by the action of heterotrophic bacteria which breakdown Hydrogen disulphide (an inorganic oil) under anaerobic conditions. This gas is formed ONLY in when there is a complete lack of Oxygen.
"Everybody is ignorant, only on different subjects." - Will Rogers