How many hours should an aquarium be lit per day? The right duration depending on the tank

You turn on the light bar in the morning when you leave, turn it off in the evening when you go to bed, and there you have an aquarium lit for fourteen or fifteen hours a day. This is one of the most common causes of algae. For most freshwater aquariums, a lighting duration of around 8 to 10 hours per day is more than enough; start shorter, at around 6 hours, for a new aquarium, and rarely exceed 10 to 12 hours. The rest of the time, fish and plants need darkness. Here is how to choose the right duration for your tank and adjust it by reading the signs your aquarium is giving you.

Why duration matters as much as intensity

Aquarium lighting is primarily for plants and photosynthetic organisms, including algae. Plants only use light if they also have sufficient CO2 and nutrients. When light is available longer than plants can use it, algae, which grow faster and are less demanding, benefit instead.

For fish, lighting sets the rhythm of the day: activity, feeding, and rest. Most aquarium species naturally live at tropical latitudes, where day and night each last about twelve hours year-round, often under vegetation that filters the light. Fifteen hours of lighting is not natural for them, and the absence of a true dark period can stress them.

Recommended durations by aquarium type

The values below are commonly recommended guidelines. They also depend on the power of your lighting: a very intense light often requires a shorter duration than modest lighting.

Remember one principle: always start at the lower end of the range, and extend the duration only if the plants require it. If you have a planted aquarium, our guide to LED lighting for planted aquariums helps you choose the appropriate intensity, which goes hand in hand with the duration.

Duration and intensity: thinking in terms of a "light dose"

For a plant, what matters is not only the number of hours, but the total amount of light received during the day: a given intensity multiplied by a duration. A powerful LED light fixture switched on for 8 hours can provide more light than an old fluorescent light switched on for 12 hours. This is why switching from old lighting to a recent LED light fixture so often results in an algae outbreak: the schedule stays the same, but the daily dose has increased significantly.

If your light fixture or controller allows you to adjust the intensity, use this setting together with the duration. For a low-demand tank, reduced intensity for 8 to 9 hours often provides better balance than full power for 6 hours. For a heavily planted tank with CO2 injection, you can instead increase the intensity while staying around 8 to 10 hours. In all cases, when changing lighting equipment, start with a shorter duration and increase it gradually, as you would for a new aquarium.

The depth of the tank also matters: light loses intensity as it passes through the water, and plants at the bottom of a tall aquarium receive much less than those near the surface. Rather than extending the duration for them, which would mainly benefit algae near the surface, it is better to choose plants suited to the available light.

The case of a new aquarium

During the first few weeks, the aquarium is not yet balanced: the filter bacteria are establishing themselves, the plants are taking root, and they are still consuming only small amounts of nutrients. This is the period when algae appear most easily, particularly brown diatoms and suspended green algae.

A cautious approach is to start at around 6 hours per day, then add half an hour each week until reaching 8 to 10 hours after one or two months. If algae take over during this increase, return to the previous level for one or two weeks before starting again.

Should you take a break at midday?

Some aquarists use a “siesta”: the lighting is turned off for three to four hours in the middle of the day, then switched back on in the afternoon. The idea is to limit algae while still enjoying a lit aquarium in the morning and evening. Opinions are divided: the break has not been shown to work consistently and may disrupt fast-growing plants. However, it can be useful in a living room where you are only present early in the morning and in the evening.

If you try it, count the total lighting duration, excluding breaks, and keep it within the recommended range. Ten hours split into two periods are still ten hours.

Daylight counts too

An aquarium placed near a window receives natural light in addition to its light bar, sometimes a lot of it. Direct sunlight, even for just a few hours a day, is often enough to cause algae on the glass or green water, regardless of the duration set on the timer. In this case, there are two solutions: move the aquarium, or significantly reduce the duration of the artificial lighting and cover the most exposed pane with an opaque background.

An aquarium in direct sunlight cannot be adjusted with the timer: adjust its placement first, then the duration.

Signs of lighting that is too long... or too short

The aquarium quickly tells you if the duration is set incorrectly. Here are a few signs to look out for:

Before changing the duration, check the other possible causes of algae: excess food, insufficient filtration, water changes that are too infrequent, or an overstocked aquarium. Our article on getting rid of algae in an aquarium covers these points one by one. Light is often the trigger, but rarely the only cause.

Schedule the lighting: consistency above all

Lighting that is turned on and off manually necessarily varies from one day to the next, and weekends or late evenings extend the duration without you realizing it. A timer solves the problem: fixed schedules, consistent duration, and an aquarium that continues to function normally when you are away, as explained in our article on managing an aquarium during vacations.

Set the lighting period to the times when you enjoy the aquarium. If you are away all day, lighting from 1 p.m. to 10 p.m. makes more sense than lighting from 8 a.m. to 5 p.m. Controllers with gradual sunrise and sunset also prevent the abrupt transition from darkness to full light, which can frighten fish. Beware of factory settings: some controllers come with a long default period (for example, from 7 a.m. to 9 p.m., or fourteen hours), which should be shortened.

And at night: do you need moonlight?

Small blue LEDs known as “moonlights” let you observe the aquarium in the evening and avoid an overly abrupt transition to complete darkness. They are not essential. If you use one, keep it very dim and do not leave it on all night: fish need true darkness to rest, and some algae can take advantage even of weak permanent light. One or two hours in the evening are more than enough.

Nocturnal species, such as many catfish and loaches, come out precisely when the light goes off. A clearly defined dark period is essential for seeing them feed normally.

Adjust without changing everything

Plants and algae react slowly. Adjust the duration in half-hour increments and wait at least a week before judging the result. Changing the duration, intensity, and fertilization at the same time makes it impossible to know what worked. Note your settings: this prevents you from going around in circles and makes it easy to return to a stable situation.

Finally, remember that fluorescent tubes age if you still use them: their spectrum and intensity change over time, which can unbalance an aquarium. LEDs age much more slowly, but their intensity, often high, means you should not extend the duration without a reason.

Also read

To choose the intensity that matches the duration: Which LED lighting for a planted aquarium?.

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Frequently asked questions

— How many hours should an aquarium be lit per day?

Generally 8 to 10 hours for a freshwater aquarium, 6 to 8 hours for a sparsely planted aquarium or a new aquarium, and rarely exceeding 10 to 12 hours.

— Can you leave the aquarium light on all night?

No: fish need a genuine period of darkness, and continuous lighting encourages algae. A very dim moonlight LED can remain on for an hour or two in the evening.

— Does lighting for too long cause algae?

It is one of the most common causes: plants cannot use all the available light, and algae benefit from the excess.

— Should you take a lighting break at midday in an aquarium?

It is not essential, and opinions are divided. If you do it, keep the total lighting duration within the recommended range.

In summary

• 8 to 10 hours of light per day suits most freshwater aquariums.
• A new aquarium starts at around 6 hours, then the duration is increased in half-hour increments.
• Daylight and direct sunlight add to the lighting: location matters.
• A timer ensures fixed schedules; check the default programmed period.
• Make adjustments slowly, one thing at a time, while observing the algae and plants.

Last updated: October 3, 2026.