You got lots of options here. I dont know what size tank you have or what CUC you are currently using and that has a considerable bearing on what I would recommend.
In a large tank, a Tang of any one of about 4 species would be your best bang for the buck depending on the fish you already have. In a Nano you might be better off with some snails of one sort or another. It just really depends on what you have, and how big the tank we are talking about is. Does this tank have corals too? If so what type and size.
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