1 Answer

They half sleep. They do not go into a full sleep like humans. 

It’s generally accepted that fish have a designated period of ‘rest’ in which their awareness of their surroundings substantially decrease. However, fish do not enter a period of unconsciousness in which they are completely unaware of the world around them. The brain waves of most species of fish are unchanged during periods of ‘rest’, which means that fish do not experience REM sleep. During this period, the fish may drift with little or no fin movement around the bottom of the tank. Most ‘sleeping’ fish rest on the bottom of the tank, though are easily roused from their period of rest if subjected to external stimuli. http://petcareeducation.com/fish/do-fish-sleep/