This is mainly caused by bacterial and fungal infections. The fins will look like torn and fragmented.
vigorous fighting of big and small fish may also result in condition that look like fin rot. It is better to carry out treatment for the sick fish in a hospital tank.
We have to treat this disease by understanding of actual underlying cause. it means the disease may be due to bacteria or parasite or fungus. so, we give comprehensive treatment it works better to heal the fish.