Fin rot is where your goldfish’s fins start to look torn and ragged, as if they’re “rotting away”. It may look like chunks or slices of your fish’s fins are missing, your fish may rub up against the side of the tank and against decorations, and you may notice a red or white outline on the ragged edge of the fin.
Fin rot isn’t a disease. It’s a symptom of some other disease or problem with your fish.