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Do Fish Need Light
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Do Fish Need Light? Yes, and They Need the Dark Just as Much

Fish need a steady day and a truly dark night, not constant light. Learn how many hours to run your tank light and what plants change.

Jack·7 min read·May 22, 2021·Updated October 14, 2026

You switch off the lamps at bedtime, and the aquarium is still glowing in the corner. You wonder whether the fish would rather it stayed on. They wouldn't. Fish do need light, but mostly as a clock: a regular stretch of daylight followed by a dark night keeps their sleep and body rhythms on schedule. Light around the clock works against them, and even a faint glow at night can. Here's what that means for how long the light stays on, how dark the nights should be, and what plants and colored bulbs change.

Light is a clock, not just a lamp

Most of what scientists know about fish and light comes from the zebrafish, also called the zebra danio. It is a common pet-store fish and the standard laboratory fish. Labs keep zebrafish on a fixed schedule, usually 14 hours of light and 10 hours of dark. One study of aging and body clocks held adult fish at about 400 lux during the day and under 1 lux at night. For comparison, 400 lux is roughly a well-lit room, and under 1 lux is close to true darkness.

The schedule matters. Zebrafish are active while the light is on and show sleep-like behavior in the dark. Researchers raised larvae in constant light or in constant darkness, and those fish showed no daily rhythm in their movement or their sleep at all. So the useful thing light gives a fish is the change between day and night, not brightness as such.

Fish really do sleep

Fish sleep looks different from ours, but it is sleep. Zebra danios at night droop their tails and hang almost motionless. Researchers at Stanford found that when the fish were kept awake, they made up for it with extra sleep afterward, the same "rebound" humans show after a bad night.

Light hits fish harder than it hits mammals. In zebrafish, light strongly suppresses sleep, and fish kept in constant light barely sleep at all. A study in Nature went further. It looked at brain activity in larval zebrafish and found patterns resembling both slow-wave sleep and REM sleep. The authors concluded that the basics of sleep evolved at least 450 million years ago. A tank light left on overnight is not keeping your fish company. It is keeping them awake.

Why a dim night light is a bad idea

Many tank lights come with a blue "moonlight" mode, and some advice says fish shouldn't be left in total darkness. The research points the other way.

The hormone melatonin rises in the dark and helps run the body's nightly cycle, and fish are extraordinarily sensitive to light that suppresses it. A 2019 review of light-at-night research reports that melatonin suppression in fish starts at around 0.01 to 0.03 lux. In sensitive humans, the figure is about 6 lux, a few hundred times brighter. In one experiment, European perch kept at night under 0.01, 0.1 or 1 lux all had less melatonin than fish kept in the dark. Even the dimmest level had an effect, and brighter light had more.

Small light sources can matter too. A 2016 paper in the journal Zebrafish reported that failing to give lab fish complete darkness can disturb their body clock. Fish housed under a red exit sign produced roughly half as many embryos.

Moonlight modes are popular, and there's no sign they are an emergency. But no study shows that fish need them, and the melatonin research suggests the best night is a dark one. If your tank sits in a room where lamps or a TV stay on late, it helps to put the tank light on a timer and keep the room's evening light away from it.

How many hours the light should be on

Labs give zebrafish about 14 hours of light because they are studying the fish, not fighting algae. Home tanks have that second problem, so hobby guides suggest shorter days:

  • Fish-only tanks: about 6 to 10 hours of light a day
  • Planted tanks: about 6 to 8 hours a day
  • New tanks: start at 5 to 6 hours and build up to 7 to 8 hours over four to six weeks

These figures come from retailers and experienced hobbyists, not from controlled studies, so treat them as starting points. They fit well with what the fish themselves need: one guide notes that most freshwater species do well with 8 to 12 hours of darkness a day.

The reason to keep days short is algae. Too much light is linked to outbreaks of green algae, hair algae and cyanobacteria (the slimy "blue-green algae"). Constant light can also stress live plants. If algae are taking over, cutting an hour or two of light is one of the easiest fixes to try. A cheap plug-in timer keeps the schedule identical every day, which is what a body clock depends on.

Live plants change the equation

Plastic plants don't care about light. Live plants do, because they photosynthesize, and they are what decides how strong your light needs to be.

Planted-tank keepers measure light in PAR, the amount of light plants can actually use. Sources disagree on the exact cutoffs, so these tiers are rough. One guide calls 1 to 20 µmol low light, 20 to 50 medium and anything above 50 high. Hobbyists on the UK Aquatic Plant Society forum put low light at about 25 PAR or less and high light somewhere above 75, or "well over 100."

More light isn't simply better. High light grows plants faster, but it also brings more algae, can burn plants, and usually needs added CO2 and fertilizer to stay balanced. That's why beginners are generally steered toward low-light plants, which need less of everything and forgive mistakes.

Does light color matter?

It can, but not the way aquarium marketing suggests. The studies here come from fish farms rather than home tanks, and they disagree from one species to the next:

  • Atlantic salmon exposed to high-intensity blue LED light showed a rise in the stress hormone cortisol, and in blood sugar, within three hours. Levels returned to normal after 24 hours. White light and dimmer blue light didn't cause the spike.
  • In the South American catfish Rhamdia quelen, researchers concluded that green light "seems to be the worst alternative" for indoor tanks.
  • Koi kept under yellow or red LEDs had significantly higher cortisol and blood sugar over 90 days.

No single color is calming for every fish, and the idea that blue light soothes them isn't backed by this research. Bright blue actually stressed the salmon. Plain white daylight-style light is a safe default. Color also doesn't change your water's pH. A hot fixture can warm the water, but that's a temperature issue, not a chemistry one.

Why LEDs are the usual choice

Fish don't need LEDs in particular. What they need is a steady schedule. LEDs just make that schedule cheaper to keep. They add less heat to the water than T5 fluorescent or metal halide fixtures. Manufacturers say they use up to about 90% less energy than fluorescents, though that figure comes from sellers and should be taken as approximate. Many also have built-in timers, which handles the most important part of tank lighting for you: the same hours of light, and a properly dark night, every single day.

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