I noticed long before I put him in the big tank. But lately he's been getting fatter.

Also, his mouth is sort of deformed. It's opened really wide and never closes all the way. Could this make him eat more and get fatter? He's never had a problem with his mouth.

Does it look like he has uniformly filled out, or is it all in one spot? Maybe he's just healthilly filling out.

He's been getting wider in all areas, but his front belly area is really rounded. It doesn't seem to affect his swimming at all.

What big tank did you put him in? Is he in it with alot of other fish? How much/what/how often are you feeding him?I have seen Betta's mouths looking somewhat odd before. I've always assumed it was because of the viscious way that they attack their food.I've seen mine actually hit their mouths on objects while doing it. There are also some diseases that will cause that too,so keep an eye on him for sure.Betta's run into problems when you see them getting fat.Whether it's overfeeding or whatever,I think that you may want to starve him a day or two,and then feed him a little pea,just incase he's constipated. Let us know how things are going.

I put him in the 29 gal with 8 goldies, 1 pleco, and 1 rosy red. There is a good filter and a buried airstone on the oppostie side. He swims around all the time. When I feed them, I put the goldies' food in one side and his food in the other. I do it that was so they won't eat his. His mouth has always been deformed. I adopted him from my little sister and it was like that when I got him. She took horrible care of him; way too much food, filthy water, no sunlight. He didnt' know what to do when I took him. But it's always been that way and I take good care of him. He's my little old fish-man.

wow I didn't think bettas were compatible with goldies!

Is it fat the way fancy goldies get fat? *thinks golfball*

Sort of. Just a mini version.