I would try Jungle parasite clear since it seems its internal (whatever it is). Medications like clout or quick-cure treat external parasites like when anchor worms begins to poke out. There is a possibility that what you have is lymphocystis which is as a result of a viral infection. It wont kill your fish unless its in the gills causing it to suffocate. If it is that, you can surgically remove it, but the killer is the possible infection after removal.