a long slim fish with blue and
      black dots on its sides

Scrawled filefish

( Aluterus scriptus )

It is a relative of triggerfish. Its trigger fin is longer and thinner. Each scale of the fish has small hard thorn. So the skin seems a file or sand paper. Normally Aluterus swims alone. I never saw a shoal or even two filefish together.

a long slim fish with
      blue and black dots on its sides