Just leave him in there. It is very important that you keep the water clean at this point so that you can reduce the chance of a secondary infection on the tail. It will grow back, don't panic. Keep an eye on how the other fish treat him. If he was bitten once, chances are it will happen again. If you see aggressive behaviour then you have to be ready to make a decision. He may have to be taken out or the aggressor may have to be taken out. It may not be because of another fish, he may have fin rot. Now before you rush out and start putting meds in the tank, perform a few water changes first. You should also test the water to make sure that isn't the problem. If you don't have a test kit, you need to go buy one.
