It might be a sign of swim-bladder issues. Are they floating near the top as well? Does it seem like it's having problems moving around? Try feeding the fish a pea! :) You posted this in gold fish,...
Swim bladder problems are common in fish, with some species being more prone to it than others. This often causes fish to swim upside down or erratically.
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your fish is probably constipated. you need to starve him/her for about 2 days and then feed them ground up peas out of the shells. epsom salts also act as a laxative so you could put some in the...