Like the ocean surgeonfish but with about ten faint dark bars down the body, which are the only reliable difference in the water.
To about 39 cm, commonly 35. A grey to brownish-grey oval disc. The mark is about ten narrow, well-spaced, darker grey vertical bars down the side. They are subtle and the fish can switch them on and off, but no other Caribbean surgeonfish has them. There is a pale ring around the socket of the tail spine and a pale band at the base of the tail.
The ocean surgeonfish is the pair that matters, because the two school together constantly. One check settles it: this fish has the ten vertical bars and the ocean surgeonfish has none, showing yellow lines radiating back from the eye instead. The blue tang is uniformly blue as an adult and bright yellow as a juvenile, and causes no trouble once seen.
It grazes filamentous algae on reef, rubble and seagrass between 2 and 25 m and usually between 2 and 15 m, and it has a gizzard-like stomach for dealing with sand mixed into the algae. It forms mixed grazing schools with the ocean surgeonfish and the blue tang, and the reason is that a school overwhelms the damselfish whose algal territories they are feeding in. The tail spine is hinged and sharp; it is a handling risk only.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.