Some kind of stress usually. When they can't breathe and stay near the top of the water, they usually have rapid breathing. They also do this when they are being terrorized by another fish in the tank. I'm sure there are other examples.
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They are also symtoms of diseases such as skin & gill flukes, fungal gill rot, Ammonia & Nitrite Poisoning, Poisoning in general, Slime diseases and Velet.