How do you cure a fighting fishes' tumor?

Tumours develop in fish when their water quality has not been good enough for a considerable time. I assume that you have not been observing the basic rules of fish keeping. If you clean up your act now it is possible the fish will recover by having reasonable living conditions. The basic rules are. :- 1 inch of fish needs at least 1 gallon of water. (a Betta is a 3 inch fish). Every tank needs a cycled filter running 24/7. Every tank needs to have at least 50% of its water replaced every week. If you observe these rules above you should have few problems keeping fish. If you don't, I can guarantee you will have constant problems with sick and dying fish.