Betta prob

Hello,

My usually healthy, happy betta seems to have something wrong. I am not
sure what the problem is, but his back-side seems to be (what I can only
describe as) "rotting away". A little of his tail is missing from around
there and he has what seems to be an injury or chunk taken out of his
back-side. He spends most of the day at the bottom or at the side of the
filter. That being said, when you walk in the room, he's straight up the
top begging for some food. He'll still go after the little pleco when he
sees it move as well. Water quality is fine and the other fish is doing
well. I have been using some anti-fungus/anti-fin rot treatment in the hope
it would get rid of whatever was causing it/help healing, but the situation
doesn't seem to be changing. It's something that appears to be getting
worse rather than better. I don't think he's injured himself on anything in
the tank from what I can see. Can anyone help?

Thanks in advance.