There are several tang species with dark spots posteriorly on the dorsal and anal fins. It may well be Ctenochaetus binotatus (Twospot bristletooth/tang), but it may be one of the other species, such as Acanthurus nigrofuscus (Brown Surgeonfish). You should be able to narrow it down to genus by looking at the teeth when it is feeding on the algae on the glass - Ctenochaetus have very fine, brushlike teeth.
Tony