Fish do sleep, but their sleep looks different from mammal sleep. How fish "sleep": they enter a low-activity state with slower breathing and reduced response to stimuli. Most fish hover near the substrate, in caves, or among plants. Reef fish often wedge themselves into rockwork crevices. No eyelids: fish lack eyelids (except some sharks), so their eyes stay open during sleep. Day vs night sleepers: most aquarium fish are diurnal (active during light, sleeping in the dark). Loaches, plecos, and many catfish are nocturnal - they hide during your light period and feed at night. Lighting matters: abrupt light changes stress fish; use a 30-minute sunrise / sunset ramp on programmable LEDs. Leaving lights on 24/7 prevents proper sleep and triggers stress, fading colors, and disease susceptibility. Maintain a 10-12 hour dark period.
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