Definately not a good idea. I personally don't have experience with the Powder Brown Tang and I don't know which "Blue Tang" you're talking about. The Blue Tang can be a Hippo Tang, an Atlantic Blue Tang, or the Powder Blue. In any case, I have experience with Hippo Tangs so I hope that's the one you're talking about. Hippo Tangs are very susceptible to ich. If you can get the Hippo to eat veggie foods from the beginning, you're fine. If the ich isn't disappearing, you've got problems. Also, the Hippo Tang can get pretty territorial in a 75 gallon. It will dominate other fish in the tank and will not allow newcomers to adjust easily.