: Do fish need a dark environment at night? Is it bad if I accidentally leave the lights lit on a fish tank overnight? Or do the fish not care whether it is light or dark?

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Fish do care if it´s day or night: they have a cicardian rhythm and many physiological events are related to the day / night time. Light could interfere with that. Source: this book, chapter 6.

Even blind fish can have a cicardian rhythm

Leaving the aquarium light accidentally lit for one night won´t instantly kill them but will cause some stress.