You could totally get away with the naso for a while. The tang police will haul me off in chains for this, but according to this article,
Estimating the Need for Swimming Space for Aquarium Fishes
This article explores required aquarium sizes for marine fishes
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you could fit one up to 11.5 inches in there. It won't work forever, but it'll have plenty of time to eat every piece of bubble algae in there.