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Cliff - I did go to an aquarium store today, and observed many species very closely. Even on VERY close inspection I could barely detect any mouth or gill movement associated with breathing, even with angelfish. The only exception was a freshwater puffer that was very active, and did have a little gill movement.
I'm thinking it may be a function of how much oxygen is present in the water. As long as there are a lot of bubbles, the fish seem to get enough O2 without any gill movement.
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