Concern about Anaerobic spots

Hey guys, thanx for answering. For once someone didn't have fish problems, a sour substrate, some funky smell, and automatically go the "H2S boiled up and killed my fish" answer. Maybe I'm wrong, but I seem to remember reading somewhere that IF a large amount of H2S was released into the bulk water, it correlated to a rapid decrease in oxygen. Is that true?, If so, why is it then that that never was to blame for the problems. The symptoms of "H2S poisoning" seem pretty similar to oxygen deprived fish if you ask me.

RTR, just curious here, what's the deal w/replacing the substrate in a planted tank after so long? I've never really kept live plants in FW before, but never thought an old substrate could become problematic.
 
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