Melissa

We have a 9 year old goldfish who suddenly, a few months back, started to develop mouth deformities. She gets blisters which then pop (after thrashing) resulting in peeling skin. It then heels and starts all over again. Water levels look ok- no3 20, no2 0, ph 7.5, kh 180. Gh is 0 bc we have a water softener however we have always had it and never seemed to affect her. I tried aquarium salt and also API fin rot but has not helped. Any help would be appreciated. I don’t want her to suffer.