I would be interested to see how this progresses. Have a Betta that looked like that for some time. Thinking the same thing, looked like it was going to explode. Went on for some time, Betta acting fine. Then one day it started to break open. Now the the poor thing has this big white patch on its side. It has been acting pretty much fine. I tried melafix but didn't touch it. I have searched the net for pictures of "tumors" and parasites ect... but can't find anything remotely like it. It isn't red on the edges or infected looking. He has a real difficult time turning in that direction and is now starting to mostly hang on the top like he is uncomfortable, but for the longest time was acting just fine. He is a very old tough Betta : ( He was in a community tank for the longest time before I took him out to QT and none of the other fish show any signs of it. I am assuming it is a tumor.
