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Try not feeding the fish for a few days. You can either isolate it with a breeder basket for that or you can just fast the whole tank (fish are fine for quite a while without food). If it's digestive-related, then it might improve. If it doesn't improve and there are no signs of systemic infection, which typically manifests as red streaks on fins/body or loss of color, etc., then I don't know that there's a lot else that can be done.