bitten fins will be frayed at the ends. The best way to tell is to observe them from a distance. If this is happening, then it means that the fish are not a good mix. They should be separated (almost like a divorce!). The fish being bullied should be placed in a proper fish tank with cycled filter, rock salt added at 2-5g/L and Melafix added.
How can you tell if your fish is being attacked? What to do?
