The short answer
Yes — fish need darkness at night to rest. Like most animals they run on a day/night cycle, and a period of dark lets them wind down, lower their metabolism and recover. Leaving the aquarium light on around the clock stresses fish, disrupts their rhythm and fuels algae. Give them a consistent block of darkness every night.
Why the dark matters
Fish don’t sleep the way we do, but they do rest. At night most species slow right down, tuck into cover or hover quietly near the bottom. That downtime is when they recover, and a reliable day/night cycle keeps their behaviour, appetite and colour healthy. Constant light removes that rhythm and leaves fish permanently “on”, which over time shows up as stress — faded colour, less activity and lower disease resistance. For more on stress, see how to lower stress in fish.
How much light is right
Aim for roughly 8–10 hours of light a day for a fish-only tank, a little more for planted tanks. The key is consistency, so put your light on a timer and let it run the same hours every day. That gives fish a predictable rhythm and stops the tank drifting into the long, bright days that algae love. For the full breakdown, see how long to leave your aquarium light on.
The algae connection
There’s a practical reason too: algae thrive on excess light. A tank lit 24/7, or one sitting in a sunny window, will green up far faster than one with a sensible photoperiod. Cutting the lights at night is one of the simplest ways to keep algae in check — no chemicals needed. Whatever you decide, avoid leaving the light on overnight: it helps neither the fish nor the glass. If you’re setting up lighting, browse our maintenance and setup guides.