Betta fish?

I just got a betta fish and have had him about two weeks. A couple days ago he started to not eat anymore and he'll just sit on he bottom of the tank all day. I've been feeding him dry pellets and today tried giving him some frozen brine shrimp but he just wouldn't eat. He also was in a fight a couple of weeks ago and I've been giving him Bettafix remedy to help heal his fins. Any idea why he's acting this way? Thanks!