Why fish cannot sleep? Because the mattress and pillows are wet
Silly question but need a detailed explanation, does the fish have pillows and mattresses? Of course not, fish is not human need on the mattress and pillow. You who watch the fish in the aquarium will think the fish never sleeps because he is always active and his eyes are not covered by eyelids. In my opinion, fish do not have eyelids like other creatures. It turns out your guess is wrong.
Fish are also creatures that have instincts like other animals, need to eat, sleep, and breed. The characteristics of living beings also apply to humans. However, the ways and techniques are different. Fish that we know that have no eyelids except sharks have a unique way to sleep. Fish whose living habitat on the reef will use them to hide for sleep, sleep is not as good as our sleep, fish need to keep watch not to be eaten by other fish. Fish that live in loose waters like tuna and similar fish sleep by not swimming, but it does not move and let itself carried by the flow of seawater. That's the life of a fish.
So, although the mattress and pillows are wet the fish will still sleep in a different way with the way living creatures on land.