I have a 2.5g divided tank with 2 male bettas that don't really seem to care about each other, the tank has been set up for a few days expecting the fish and last night they were finally put in.
all night they were swimming around and seemed to like it, but this morning one of them was laying on the bottom of his side of the tank and started leaning to one side.
I read that this might be swim bladder related and could be caused by overfeeding, we fed them once last night before bed, and again this morning around 8am. about 2-4 pellets each.
the water was treated and left standing for about 2 days before adding the fish, and the tank has a small filter (all came in a aqueon betta starter kit) that doesn't seem to bother the healthy fish on the filter side of the divider.
I don't have a heater but I plan on going out to get one today, even if it isn't necessary for a tank this small, I still need one to start my 10g this summer when I'm home from school.