My male betta has had a tumor growth on his side for the past few months. It's been getting bigger now. I'm pretty sure it's not dropsy. I clean his whole bowl every 3-4 weeks, and I feed him Aqueon betta pellets. His behavior has been normal, and he doesn't act sick.
....woah woah woah woah!! You clean his bowl every 3 to 4 WEEKS!? O__O;
In a BOWL, you need to be doing DAILY water changes! It looks a lot like ammonia poisoning to me, more than a tumor. Ammonia burns are due to the ammonia build up in your bowl/tank.
May I suggest you either 1) clean his bowl 50%-80% every day and then do at least one 100% change a week or 2) get him a bigger tank (2.5gal at least) and do one 50% and one 100% each week.
It's my fault for being lazy and not cleaning his bowl. I would upgrade him to a larger tank, but my parents won't let me upgrade him to a 5 gallon because I already have a 38 gallon aquarium for my orandas, a 5 gallon tank for my other male betta, and another 1/2 gallon vase for my female betta.
I also tested the water, and I agree it must be an ammonia burn.
yeaahhh that would be it. Hmmm what if you divided the 5 gal.? That way each betta would get 2.5 gals each? I house my two males in a 5gal together (with a divider of course!) They're easy to make too! I just bought those plastic essay holders you use for papers for class and punch some holes in it and cut to fit! :D
^__^ That way you'd have to do less water changes. Hope this helps!