My beta fish live in a 2 1/2 gallon acrylic tank, i don't know the temperature but their is a filter. There aren't andy airstone's or aeration. There is a heating light on the top of the tank and the betas live in the tank together with a divider between them but no other fish. I feed them Tetra Bettamin Tropical Medley and feed them once a day. I just recently changed the tank they were living in and plan on changing the water once a month, but before that i had changed the water (all of it) once every two weeks and that was in tanks without filters. I had changed all the water in the previous tank, don't know about this one will probably do the same thing. I have not tested the water and don't know how to. My fish has changed in appearance, he now has a white spot by his tail fin (looks somewhat like a scab) and has a very fat stomach. His behavior has changed by him not moving around as much and stays on the top of the water, also doesn't eat much anymore. I started noticing these symptoms ayer my other beta fish got into its tank four days ago, there were signs that they had been fighting but had not been when i found them. I haven't been treating him and i don't know what to do for him. He hasn't been sick before and is approximately 6 months old.