I have 3 spotted puffers, one of which is sick.
I've tried putting him in a tank alone, medicating him, but nothing works. He started with one spot of skin that looked like he got scratched.. but it has gotten to the point where most of his skin is gone, his tail is shredded and he has lumps on his belly. I'm not sure if he's been eating much, he is smaller than the other fish in the tank... The other fish are all fine, so I have no idea what is wrong with him. Please, anyone who can help!!

I would recommend putting him in a separate empty tank so he does not get scratched any further. At this point ,I don't think putting medicine would help. Just makes sure he eats a little more but not to much to give him energy to heal. The only thing I think you can do is let him self-heal. hope this helps