I've had my betta for about 3 months and he's been active and swims around a lot. I have him in a 10 galon tank with filter and it's heated at 80 degrees. The other day, I got a pleco and added him to the tank but he died the next day. I think he was just sick, figures, it was from petco. I got another one to replace him, but ever since, my betta hasn't been swimming around much. He sits at the bottom and doesnt move. He has big beautiful fins that were always so flow-y but now he keeps them tight to his body. Please help me, I don't know what's wrong. What should I do?