Hippo Skin Infection Options

Alien Reef

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I have a client that has a very old Hippo Tang that has what seems to me to be a nasty skin infection. The water quality is not great and he is getting out of the hobby. I want to save this fish which I believe is 6 years old. I have a variety of medicines including focus to bind it to some frozen food which she is still eating. Should go with metroplex, Kanaplex or triple sulfa? 37F8F4CF-A4BB-4164-97E8-347DE8C16D16.jpeg
It may be just the photo but is the stomach sucked up? Looks flat. Again might be the photo