While most fish rest by reducing their activity and metabolism, fish "sleep" is different from that of land mammals
The nature of fish "sleep" is an area of active research. While fish do not sleep in the same way that land mammals sleep, most fish do rest.
See Also
Does Noise Scare Fish Away?Fish have feelings, new study findsFish Feel Pain | PETAHow do animals recognise their young and do they know they reproduce?Research shows that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. These periods of "suspended animation" may perform the same restorative functions as sleep does in people.
