On occasion, ratty and mangled fins are the result of a Betta biting his own tail rather than outside causes. It is suspected that in most cases, the ragged fins are a result of poor water quality or injury from tank decor or other fish. Too often people believe their betta is biting his own fins when the problem is environmental but in some cases the damage is self-induced. First, Betta owners should rule out other factors like poor water quality, sharp rocks, pointy plants, heavy filter suction or aggressive tankmates. I then recommend observing the fish closely to see if he can be caught in the act.
The reasons Bettas bite their own fins are unknown. Some say it’s stress, others say boredom and still others think it could be hunger, pent up aggression or even hereditary. The truth is, we just don’t know. There are a few things you can do to help him along. First, continue to keep your water very clean to avoid infection. Some Betta keepers have successfully broken the habit by changing the aquarium decor around or by adding more silk or live plants. Moving the tank to a new location or the fish to a new tank may help. Adjusting the light levels may be useful too reduce reflections which may fuel your Bettas aggression. If your aquarium is large enough, you may find adding a few peaceful community fish will keep your Betta occupied.
In addition to clean water, you can also add Pimafix, Bettafix or half strength Melafix to the tank to help regenerate fin growth. If your betta reacts to the medication negatively, discontinue use.