Some bettas just like smaller tanks. Personally, I feel that 1.5 gal is the smallest I'd keep a betta in. I've got a 1.5, a 2, a 3 and a 5. I had Neptune in my 5 gallon which used to be divided. I took the divider out because I didn't like it. It was completely bare for a couple days while I waited for my plants to come in, and Neptune completely bit his tail. I figured it was because there were no hiding spots. He gets easily stressed and loses color almost instantly during a water change. I put the plants and driftwood in, some color came back and he was doing better. When my AB fish came in, I moved Neptune into a gallon bowl that is unheated (my room is warm so he's not cold at all) until I could grab him another since I wanted the 5 gallon for the new one. I put him in there Saturday. His color was alright. I left for class this morning at 845 and came back at 230 and WOW. His color is deep and rich again! He's never colored up this nice.
Moral of the story- Some bettas, with proper water changes, can be perfectly happy in a smaller tank. (Though I still don't think any fish would be happy in those .5 gallon or smaller "betta bowls")
Anyone else have a fish like this?