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?


2 Answers 2


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.

  • Why not just connect your lights to a timer that will shut it off automatically at a given time each day?!
    – martin
    Oct 9, 2013 at 18:56
  • 1
    Also leaving the lights on for long periods can increase algae growth - which, as well as being unsightly, can decrease oxygen levels in the tank. Jul 18, 2014 at 12:41
  • 1
    As an addendum, the night is also required for some plants. Compare for example this question.
    – Voitcus
    Jun 2, 2015 at 10:46

I would set a light with a timer, fish are less active when the tank is dark and can get stressed, which means they are more vulnerable to illness.

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