Most fish that live in habitats where gases are likely to bubble out have alternative breathing systems. Think anabantoids like Bettas - good Thai fish with survival skills via an ability to breathe surface air when things get bad.
I stepped into muddy stream bottoms in Gabon, and watched the mud flow away at a pretty solid speed. The small fish we were catching had somewhere to go if an elephant decided to take a dip (or a Canadian stuck his big feet in their habitat). Pick a point in an average tropical stream and think of how many litres flow over it on an average day. You'll probably find many thousands more than any plastic filter or even water changes provide in a tiny glass box.
Anaerobic sections in tanks are interesting. They can be very deadly to bottom dwellers like the Cory group. If the tank is large enough and well filtered enough, the fish can dodge the poison gas. Interestingly, some species are very aware. Dwarf Cichlids leading their young fry around a tank have been shown to avoid areas of low oxygen, and to keep their fry away from them.
With poor maintenance, you can suffocate fish easily in a small tank, because they have nowhere to escape to. What happens in nature and in tanks tends to be different. The glass walls...
There can be large scale fish die offs from gasses though. Volcano crater likes are well known for this, although some of their residents have evolved interesting red blood cells to resist the effects of low oxygen.
