Do fish need a dark environment at night?

Question

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?

Answer 1

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.

Answer 2

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.