Is it Better to Leave Aquarium light On or Off?

Aquarium light should not be on or off all the time. Fish need period of light for activity and period of darkness for rest. If tank has continuous light or darkness, it can have detrimental effect on fish health and ecosystem of tank.

Is it Better to Leave Aquarium light On or Off

So, long should I leave aquarium light on or off?

This depends on several factors but generally it is best not to leave aquarium light on for more than 12 hours per day. If you have aquarium lights on during the day then turn it off during the night or vice versa. This will mimic day and night schedule of fish natural aquatic environment. You can also install a timer to set the time to turn on and turn off light.This will make lighting duration precise and consistent.

Type of fish and plant

If you have tropical fish and plant then leave aquarium light on for 12 hours per day.

If you have cold water fish and plant then leave aquarium light on for 8 hours per day.

If you have only fish and no plant then leave aquarium light on for 6 hours per day.

Tropical fish and plants live in environment where they get 12 hours of daylight and 12 hours of night. So to stimulate tropical conditions for your fish and plant, leave aquarium light on for 12 hours daily.

Cold water fish and plant in live in temperate zones environment where daylight they get is not uniform . It fluctuates with season. To recreate temperate zones condition, adjust aquarium light hour according to seasonal daylight hours.

It is advisable to keep plant and fish that are found in the same natural environment. Tropical plant with tropical fish  and Cold water plant with cold water fish.

Growth of algae

Algae will multiply rapidly if there is too much light. But before you shorten the duration of light you need to remember that excess nutrients can also cause algae growth even in low light. To fight algae growth, cut back on nutrients and lighting duration but not both at once. Decrease either nutrient or lighting first and observe the result which may take couple of weeks. If you don’t notice any changes, reduce the duration further but not drastically because plants will die in low light.

Location of aquarium

If your aquarium is placed in a room or near window that gets lot of of natural light then you may need aquarium light for 5-6 hours per day only. But in winter you may have to turn on aquarium light bit longer due to shorter daylight hours. However never place your aquarium where it gets direct sunlight. Algae tend to grow uncontrollably in direct sunlight.

Temperature of water

Aquarium light can increase the temperature of water to a dangerous level. If let uncheck it can kill your fish and plant. It is recommended that you use aquarium light that produces cooler light such as standard fluorescent light and LED.

Aquarium lights like vho-fluorescent and metal halide have their own advantages but they produces lot of heat. This raises water temperature significantly. Do not use it for longer period of time if you use these type of lights and monitor water temperature frequently.

Why it is better  to leave lights on?

In addition to natural light, aquarium light provides energy to plants and fish.

For plant, aquarium light is vital source for photosynthesis and growth. Without the adequate lighting, they can not synthesize food from carbon dioxide and water and will die eventually.

Fish have less biological need for light than plants but it is just as vital to them.

  • Lack of light can stunt growth in fish.
  • Bright light regulates the pigments enriching the color of fish.
  • Improves their ability to fight disease.
  • Regulates metabolism

Why it is better to leave lights off?

Most aquarium fish are active during day and sleep at night so if you leave aquarium light on at night, it can be very stressful for them. Some fish like zebrafish need darkness to sleep. They will stay awake if lights are on.

Fish need their daily sleep to stay healthy or they can easily suffer from depression and sleep disorder. Lack of sleep can make them lethargic and even aggressive towards other fish. Loss of appetite is another harmful effect of sleep deprivation.

Another reason to leave lights off is to avoid algae problem. Plant and algae compete for nutrients. If you don’t curtail algae growth, algae can easily outcompete plants as they  photosynthesize constantly in light. Plants can not thrive in low nutrient ecosystem.

Algae bloom also depletes oxygen in water which can kill fish in your tank.


Leave your aquarium light on and off so that your tank has both day and night environment.  Fish and plants don’t get experience 24 hours of light or darkness in nature. They will only thrive if your tank setup match their natural habitat.

Invest in timer. There are many affordable timer available.

Leaving light on 24 hours will result in unnecessary consumption of electricity too.

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