Sick Beta, rest of tank is fine?

Something is up with my beta and I can't figure out what. I cleaned his tank entirely recently, and he was fine after that... he was fine after I got my 3 tetra, though one fell prey to the filter WHICH IS NOW FIXED! He was fine the day I brought two tank decorations and some species of snail home... but now he's ill. I woke up the next day and he was laying around, not very active, and the top of his back fin is shredded. The new tank decorations and hiding spots I got are either solid with no ridges, or plant like but soft/spongy... the two tetra have seemingly taken over the tank.

It's freaking me out a little. For a few days, he only came out to eat. Now I know he's still alive (I can see his fin fluttering around from beneath his tree stump), but he refuses to come out of his own accord. I had to completely pull out all decorations this morning just to figure out if the guy was alive.

I'm not sure what's wrong with him... I've only noticed two things physically different about him. The fin being a little shredded, and his "bald" patch has traveled further down his body.

Help? :(

Sounds like a territorial dispute and the Tetras are winning. Take him out and put him in his own jar or tank. Treat him with Salt in his water or Betta Revive

I have around a quart size jar that I use for him when cleaning is tank, he stayed in it for a week before I bought his tank as well. Around how much salt would I need for it? I've searched for Betta Revive for over a week now and I can't seem to find it at LFS or chain stores.

Put him in your quart jar doing 100% daily water changes with a pinch of salt each time until he looks healed up. That should help heal him. I think they are just nips from the other fish. You may want to invest in a tank just for him.

I think I'll do the opposite. I have a 10 gallon tank and a mother wanting her own tank in the living room. I'm thinking I'll set it up, throw the tetra in there with some other fish of her choice, and then give Frio his own tank again. :D Thanks for the help.