Yes fish are sleeping
Fish do not have eyelids that is why it is very difficult to realize that fish are actually sleeping! Sleep is vital for all living species, fish are no exception to the rule. To protect themselves from predators, fish adopt defensive strategies that vary for each species:
Rays bury themselves in the sand
Other fish hide under stones or caves
Triggerfish are sleeping between rocks and opens their “trigger” fins up. Thanks to this, they can stand still even with rough sea or if something is trying to grab them.
Some coral reef fishes change color at night to be less seen
Parrotfish covers its body with a protective mucus
If something breaks the its bubble it alarms the parrotfish that something is wrong and awake him
Some fish swim while sleeping
Some fish continue to swim while sleeping, such as mackerel or shark. Sharks, to breathe must swim permanently to filter the oxygen contained in the water.
Mammals sleep on the surface of the water
Finally, dolphins and whales remain on the surface of the water during their resting phases. To be able to breathe regularly, whales sleep with half of their brain active.