do fishes sleep?? how do they sleep??

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well i take care of fishes in my cute aquarium.. i'm just so curious if they get enough rest or do they even sleep??=) because i notice that they seem to swim the whole 24 hrs.. they swim from the first hour in the morning when i wake up until i go to sleep and wake up again in another morning..=) they seem to be the most tireless creatures, aren't they??=)