I've never experienced any skin/scale issues with fish when using that level of magnesium sulfate, and I'm not sure what to think about that. Either way it sounds like the skin condition has improved, which is good. As far as the nitrates, if your tap water isn't the cause, and water changes aren't lowering the nitrates by much, then the only other sources left are your filter/hoses etc, and whatever's in oyur tank, rockes, substrate, etc.
If the filter has been cleaned, check the hoses etc just to rule those out. Since you just cleaned your filter, again, I wouldn't start removing any decorations etc from the tank just yet, but you might want to consider those next. Typically large water changes will knock the nitrate levels down by the % of fresh water being added.
Seeing as your nitrate levels are sky high, you might want to reconsider how often & large of water changes you perform in that tank. If it was me, with that size of fish in a 75 gallon I would shoot for at least 60% water changes every week. What do you typically feed this fish, how much, and how often?