[Luba]

Spoiled is there by any chance (non fish owner here) that the bump is a result of a parasite of sorts. Sometimes a parasite bite or infestation can start of as one thing then lead to something else.

Do you have another tank that you can transfer the fish from this tank into another while you thoroughly clean that one out? I guess that would mean buying some sort of safe disinfectant cleaner from a pet store to clean the aquarium and the filter lines etc?? Just a thought!