Hello Plec...
Unless you have a degree in fish biology, it's difficult to know what your fish has, if anything. A little standard aquarium salt added to your water change water will kill most bacteria that can infect aquarium fish.
I would suggest adding just a teaspoon of salt to every 5 gallons of your replacement water and make sure the new water is a bit warmer than that in the tank. Increase your water changes to half the tank volume every week for a month and see if your fish's health improves.
This small amount of salt won't damage plants or hurt sensitive fish. I use it regularly in my Corydoras tanks, and they do fine with it.
It's been my experience that large, frequent water changes, with pure treated tap water and a little salt, over a short time, will help.
B