I bought him yesterday and he was in fine shape... Quite active, floating about and exploring the little round plastic container he came in. The people I bought him from sold me a little kit that came with a rectangular plastic housing and some gravel for the bottom.
When I got home, I rinsed the gravel for a good 10 minutes under extra hot water from the tap, then filled the bowl with water, letting it sit for about 4 hours to off-gas and come up to room temperature. My flat is pretty warm, probably 24C if I had to guess.
Anyway, the betta now just floats in the corner, either at the top or the bottom of the tank. I tried giving him some food, but he just ignored it. He'll stir around a bit if disturbed, but otherwise just keeps himself pressed up/down in the corners. Any ideas what's wrong?
I went out and bought some bottled water in case it was that, and refiled the tank for the most part with the new water. Still, he's not doing too much.