The short answer
No — you should not leave the aquarium light on at night. Fish need a proper day/night cycle to rest, and running the light 24/7 stresses them and drives algae growth. Switch the light off overnight and, ideally, put it on a timer so the tank gets the same predictable hours of light and dark every day.
Why not leave it on
Two things go wrong when the light never goes off. First, fish don’t get to rest. They run on a natural rhythm, and a permanent daytime leaves them “on” around the clock, which shows up over time as stress — faded colour, less activity and weaker disease resistance. Second, algae love the light. The longer and brighter the daily light, the faster algae colonise the glass and decor. A tank lit non-stop, or sitting in a sunny window, greens up quickly. For the wider picture, see do fish need darkness at night.
What a healthy schedule looks like
Aim for around 8–10 hours of light a day for a fish-only tank (a little more if you keep live plants). Consistency beats total hours: pick a block that suits when you enjoy watching the tank — say late afternoon into the evening — and let the timer repeat it daily. Avoid stacking extra hours “to see the fish”, which just feeds algae. Our full guide covers this in how long to leave your aquarium light on.
If you want a night view
If you like watching the tank after dark, use a dim blue moonlight for an hour or two, not the full bright light, and still give the tank a real block of darkness afterwards. A soft moonlight is enough to see by without disrupting rest or feeding algae. Keep it on the timer too, so it never becomes accidental all-night lighting. For more on stress and rest, see how to lower stress in fish.