What kind of fish is this ?
This is the Plates better know as batfish. Adults are rather disc-shaped fish, with laterally compressed bodies and large dorsal and anal fins that give individuals a somewhat triangular profile. They can reach lengths of around 70 centimeters . They love to hang in the currents and a a welcomed companion during the safety stop.
What's that for a fish ?
It's the red-tailed butterlyfish, Very common fish around the Similan islands. It usually swims at depths of between 3 and 15 meters. It can grow to 18 cm in length. The red-tailed butterflyfish is brown to black, with lighter scales giving it a spotted appearance. It has a prominent, vertical white streak behind the eyes, a dark stripe over the eyes, and another, smaller white stripe in front of the eyes. The base of the tail is bright red, followed by a black stripe. The tip of the tail is diffuse white. Its diet consists of coral polyps.
You can see the red-tailed butterflyfish on a Similan islands diving trip