Did you add anything new to the tank recently? a parasite can very well be on the gill to a high degree and kill the fish before it will reach the outise body and be visible.
It could also be a problem or an infection with the swim bladder, so that the effort to stay afloat and up side up was what was stressing the fish and making it breath faster or heavier?
If you did not add any new fish for a long time, that would exclude parasites and make it more likely to be a swim bladder problem.
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More fish die from human stupidity than any other disease...
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