I would be careful using medications unless you can with some certainty identify the root cause. A photo might help other members identify the issue. A white growth is unlikely to be fungus.
At 11 years this fish is getting on, as the normal lifespan is deemed to be 15 years. The high nitrates may be a contributor here. This species is intolerant of nitrate and/or poor water conditions. The habitat where this species can still be found is instructive of its aquarium needs; it lives in gaps between the rocks and its habitat is characterized by large rocks and a sandy bottom. This area is fast flown by tides and the depth of water is more than 1 meter (Kulabtong, et al, 2014).
We now know that nitrate levels tend to slowly affect fish, weakening them generally, which makes them more susceptible to various problems. Nitrate at 40 ppm is more than double what it should ever be. If nitrate in the source water is zero, it is occurring solely within the biological system and should be easily controlled. Larger water changes, or more frequent, less food, not overstocking, and live plants are all relevant factors. If some or most of the nitrate is in the source water, that requires different handling.
A35 gallon tank is small quarters for this fish, and that may have been another contributing issue. Also other fish species. These are just general suggestions, but each of them is without question something that can affect a fish.