Really? because he had the "white fuzzy stuff" at the base of his tail and I had to use melafix and primafix and and his tail stayed on for a few days, then one day it was gone. I thought that's what fin rot was. I could be mistaken though.
no no no no no. fin rot isnt white and fuzzy lol. it eats away all the edges of the fin and it turns them black also. he may have had a fungus and it was itchy and he decided to bite. that sounds plausable to me anyways.
i would keep the water extremely clean, keep a steady warm temp and add salt. this should heal the fin. if it doesnt then you still have some nasty stuff on his fin... :-( in which case you should take more pics for us to see