On a budget, too eh? I'm a member of the same club!
I'd just trim the tail off to stop the rot. Don't cut into the meat of the fish, leave a little bit of the tail there and the whole thing will grow back as long as you do it right. Then you don't need any meds or anything.
You did check the water parameters, correct? Ammonia, nitrItes, nitrAtes, pH, etc.? If the water quality is poor in the main tank, the infection will just start again anyway.