My clown is my oldest fish... made it through a lot of trial and error and others dying... Big Mac has outlived 5 Blue/green chromic, 4 bengali Cardinals and a Flame Angel.
I would be absolutely SHOCKED if it was a water issue.
I'd be shocked if a Blonde Naso stay healthy in a 75 for very long... 6-8 inches is pretty much the Juvenile size for these guys and then it's a slow growth to 12. Most top out between 12-14 but can get to 16-18 (not often but possible).