I have always thought that male spotted boxfish is the coolest looking fish out there. I have wanted to have one, and have done alot of research on them. I think you would be fine with your cowfish, since both are relatively passive fish. The problem arrises with things like triggers, puffers, and others that are super aggressive feeders. I have been trying to find one for some time now, and can only either find females, or not very healthy looking specimens. If you do get one I would love to see pics posted.
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