Unfortunately, fish TB is a slow infection, and the fish can show symptoms for a while before it becomes really bad. Definately isolate that fish, and treat with tetracycline or Kanamycin (aka Kanacyn). It really isn't sounding good for him, i'm afraid. Clove oil is the only way I will euthanize a fish, it is the easy on the fish and also on the owner.
When you pick up a tank, take a sample of your water in, and ask them to test it to see what is up in there.
I'm gonna send you a PM