05-12-2013, 08:30 PM
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Very true. It is one of those things you just learn with experience. To do this, observe the fish at different times of the day, and on several different days. Respiration is always going to be faster when eating or after eating; after more activity (swimming, spawning). The smaller the fish, the more it respirates; larger fish are slower, all else being equal.
The gill operculum is a good guide too; normally it is close to the fish, but as respiration increases due to problems especially, it will extend out, so you can see behind into the gill chamber a bit.
Sometimes the sudden change in the fish's position with respect to the filter return flow can signal problems. A fish that normally remains elsewhere but now spends all its time standing directly in the flow may be having trouble. But this can't be confused with occasionally, nor with feeding--hungry fish may do this because their instinct tells them that food is more likely to be found in the current.