No matter how good tap water is, there's probably been some amount of chlorination and that can affect a frog over time. Does your filter work on chlorine as well?

The anti-fungal treatment for fish mentioned in the link might possibly work, but keep in mind it's not the same as going to the vet and getting a diagnosis, knowing how to treat it exactly, what dosage, etc. Sadly sometimes the vet is not an option, but you can call local vets and see if anyone sees exotic animals maybe?

I'm sorry.