it is a good question and it varies by species, nesting fish, like clown fish and certain other species do actually sleep, though they are easily disturbed, and yes even sharks sleep,[they is a fish! >.>;] species of fish that have eyes covers will keep one eye open, and even while "resting or "sleeping" any changes in the current or pressure in the water will alert the fish, even changes in light will wake them, and cause them to become fully active again. saying most animals rest" is not true all animals rest and all animals sleep even if only for tiny bursts. even the crabs in my bw tank used to sleep, their eyes would lower and they wouldn't move for hours untill something changed then they would be back to activeness and hyperactiveness!
forgot to metnion sharks do sleep, and they keep 1 eye open while constantly moving, HOWEVER not everything has to be constantly moving, sharks lack certain muscles which allow them to forcully pulls water through their gils so in turn they have to continue swimming, even whikle they z z z although i wish i could explain more i feell asleep half way through the lecture