Even in huge public aquariums, 7-gills always look banged up. They have them at all the local aquariums (Monterey, Aquarium of the Bay, and Academy of Sciences), and every single 7-gill has rubbing issues.
How big is your shark pool turbo? I'd love to see pics of your lemon - I've always been a bit concerned about getting one because of their aggressive nature, but they're gorgeous when they're older.