Help! Betta Keeps Shredding Fins

Ok, I'll get a heater, but do you know of any that go out of the water? I read some reviews and some people said the heater malfunctioned and electrocuted their fish. I really don't want that to happen.
Do I need to medicate him or will it heal with just warm clean water? I would prefer to not do any medicines if that's possible, but I do have some erythromycin if that's any good.
Would 10 gallons be enough for a cory?

Thanks

Heater malfunction like everyone here said is very, very rarely. Hydor is a reliable heater (I have multiple Hydor's) -- I guarantee you, it will not malfunction. Get him a heater -- it'll cost less then $20.

Like someone suggested if you are keeping the cory, upgrade to a 10G or higher.

 

I agree with Katie13. I had a similar issue with mine and I made his tank bigger and changed the water every few days. But I also read somewhere that sometimes they do it when they are stressed.
My baby Betta shed his fins but that's because he was growing