I have a male cobra guppy in a 5 gallon tank by himself. He is active and eats well. I do weekly 75% water changes and my parameters are fine. A couple months ago he had a mild case of fin rot. His tail split in half but within a week it fully repaired itself, no medication needed. 2 weeks ago it happened again but this time he lost half his tail and it doesn't seem to be improving. There is also a small bubble, like a blister, on his tail. The rest of his fins are fine. It hasn't affected his ability to swim properly but I'm worried that eventually he will lose all of his tail. He has no other symptoms of disease. Please help!