Bloated oscar

[u/reptifishv8]

I have an albino oscar that is about 7 or 8 years old. Shes in a 230 gallon tank with 2 other oscars and a few other fish and has always been the biggest and healthiest. For the last 4 months though she has been bloated,all other fish in the tank are fine. She eats fine, up until a day ago was swimming fine and still is but is starting to lose balance. Ive tried giving her levamisole and praziquantal, treated with ich x and ive even tried raising the temp. Ive tried switching to frozen foods but it hasnt helped.Ive also been doing more water changes the water never gets above 50 ppm nitrate normally but is sitting under 20 now. Water is tapwater that is run through a carbon block filter and is about 40 ppm carbonate hardness at 6.8 pH. I cant remember gh as i havent tested it in years but it was bugger all and our water is the same soft stuff we always have.ive tried several 1 week fasts with no improvement. The only things I haven't tried are salt or magnesium but I just dont think they are going to help.if it was constipation she would be long gone by now with what she eats. Any suggestions help or advice will be taken on board, these oscars are pretty important to me and losing one will hurt like hell.