I think my Betta fish is really sick!

Question
I've only had my male betta fish for 2 months. He was gift from a friend. When I received him, he had a big lump on his belly. My friend said she checked with the store and they said that he was overfed and to diet him for a few days. During that time I gave him 2 pellets once a day. A week soon rolled by and there were no changes so I decided to feed him normally(3 pellets twice a day). About a month rolled by and he looked even fatter so I put him on the diet again. More than two weeks have passed since the diet and yesterday I checked up on him again. He looks fatter than before I started to diet him!He still eats well and he even begs for more food. He seems happy, active, and he poops enough so I don't think it's constipation. My sister thinks he has a tumor, is it possible Betta fish can get cancer? If he does have cancer what should I do? I don't think I could stand and watch him suffer and die.

Answer
Hi.
Feed him a varied diet of bloodworms, brineshrimp and daphnia to keep him satisfied. Also, give him some cucumber and boiled and shelled peas.
This may be abdominal dropsy, if so there is little you can do but wait to see if it heals.
I hope he gets better,
Kathryn