The short answer
Fish see well in low light but cannot see in total darkness — no animal can see with no light at all. What lets fish cope at night is a second sense: the lateral line, a row of sensory organs along the body that detects movement and pressure in the water. Between good low-light vision and this “sixth sense”, fish navigate the dark comfortably.
Vision built for dim water
Many aquarium fish come from murky rivers, shaded streams or deep water where light is scarce, so their eyes are adapted to make the most of dim conditions. In a home tank they see perfectly well by the ambient light of a room, and at dusk and dawn they’re often at their most active. What they can’t do is see in genuine pitch black — but a home is rarely that dark, and even faint light is enough for them.
The lateral line
Where vision runs out, the lateral line takes over. This line of tiny sensors detects vibrations, currents and the movement of nearby objects, effectively letting fish “feel” their surroundings in the dark. It’s how they avoid the glass, find cover and sense tankmates when the lights are off. Some nocturnal species, like certain catfish, become noticeably more active at night, relying on this sense and smell rather than sight.
What it means for lighting
Because fish get around fine in the dark, there’s no need to leave a light on for them at night — and doing so is actually harmful, disrupting their rest and encouraging algae. Fish need a proper day/night cycle, so switch the tank light off overnight, ideally on a timer. If you enjoy watching after dark, a dim blue moonlight for a short spell is plenty. See do fish need darkness at night and should I leave my aquarium light on at night.