fish sleeping isnt the same as terrestrial animals sleeping. i read somewhere that fish will "turn off" only one eye while the other one is "on stand by" mode, ready for danger. basically, the side of the fish that they feel there's more of a threat of occuring, that side will have the eye on stand by mode. for example, if a fish is sleeping near an ornament, and its right side is facing the ornament, then the right eye will be turned off while the left eye will be minimally taking in information on their surroundings. i think that as a result of this, only half of their brain is asleep as well. also, fish need to be somewhat awake so that they can undulate their fins and stay upright.