how long should aquarium lights be left on

How Long Should Aquarium Lights Be Left On?

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Aquariums are fun to fill with beautiful plants and fish, but are not simply decorations. They are delicate ecosystems that need proper care to thrive, and aquarium plants and animals are living organisms that need light. When lighting the aquarium, it’s crucial for your critters’ survival to note how long the aquarium lights should be left on or off. 

So, how long should aquarium lights be left on?

A good rule of thumb is 8 hours a day. Most plants photosynthesize between 6-16 hours, and most aquarium fish come from environments with the same light time exposure. The exact number of hours can vary depending on several factors like the age of your aquarium and the specific species of fish and plants.

Why Lighting is Important for Aquariums

Plants need light for photosynthesis in which they consume light, CO2, and other nutrients to create more plant matter. The byproduct of this process is oxygen. The amount of time that plants are exposed to light for photosynthesis is called the photoperiod.

The longer the photoperiod, and the more light provided to plants, the more plants will want to grow. How much they actually grow will still be dependent on the other nutrients available and overall balance of the ecosystem. All plants have a minimum light requirement to grow that is dependent on the species of plant. 

Our finned friends also need a healthy diet of consistent light to thrive. The natural environment that most aquarium fish come from will have anywhere from 6-16 hours of light. The species of fish and in what areas they are most commonly found will determine whether they need hours of light exposure on the higher or lower end of that range. 

Fish that come from tropical environments or shallow waters like rivers will be more accustomed to longer periods of light. Whereas fish that are naturally found in areas further from the equator or in deeper waters can thrive on shorter periods of light.

Should I Turn My Aquarium Light On/Off at Same Time Each Day?

Aside from the length of time that your aquarium is exposed to light, the consistency with which you stick to a set schedule is arguably most important. Maintaining the ecosystem within an aquarium is all about balance. The right amount of light, the right amount of nutrients, the right amount of beneficial bacteria and all of this can be easily sabotaged by an inconsistent lighting schedule.

Turning an aquarium light on in the morning 30 minutes late, or, even worse, forgetting to turn an aquarium light off overnight are enough to throw off the balance of that ecosystem and ruin all that you worked for. This is why timers are a no-brainer. Forget about setting reminders or stressing about getting home in time to turn a light off. Adding a timer to your aquarium light, or, even better, buying a light with a built-in timer will remove the risk of human error and keep that ecosystem in balance.

If you already have a light without a built-in timer, then an outlet timer is the way to go. We like the outlet timer by NICREW. It is super easy to program and allows you to set up 16 different schedules. On top of that, it’s built specifically for aquarium lights. You can purchase it right on Amazon – our affiliate link is below!

NICREW Digital Timer Outlet on Amazon

How to Know if Your Plants and Fish are not Getting the Right Amount of Light

Finding the right balance of light for your aquarium can be tricky. If you’ve chosen rather temperamental plants or fish it can take time to figure out the right balance. Oftentimes, our plants and fish will communicate to us if they aren’t doing well. By paying close attention to the behavior, growth rates, and visible signs we can glean an understanding.

When both plants and fish are not getting enough light they will grow at a much slower rate. There can be discoloration and fading in plants. Fish may have a lack of appetite or move lethargically. In the case of too much light, fish will sometimes hide at the bottom of an aquarium or under objects that cast shadows for elongated periods of time.

3 Key Things to Consider When Determining How Long to Leave Aquarium Lights On

How new is your tank, water, fish, plants, etc.

If you’re starting a brand new tank from scratch with new water, new plants, and new fish, then you should start off with less light exposure and work your way up to about 8 hours per day over the course of a few months. This is because your tank needs to develop good bacteria to be healthy in the long run – just like in nature. This bacteria will mainly originate in your tank’s filter. 

For the first couple of months, leave the light on for about 3 hours per day. Then, over the next couple of months, gradually increase the photoperiod up to your desired goal amount (e.g., 8 hours a day). Algae growth in the beginning will be normal due to the lower photoperiod.

Quick Tip: if you have a tank already and want to start a second tank, put a second filter in the old tank and leave it there for a month or two to grow bacteria. Get a headstart on your new aquarium by taking that secondary filter out of the old tank and putting it in the new tank. We tried this and were thrilled with the results. We didn’t have any algae to deal with in the new tank (algae is normal when starting an aquarium) since the bacteria already developed in the filter while it was in the old tank. 

If your tank is already established with existing water, fish, and plants, then you’re looking at that 6-16 hour span for the photoperiod. The exact target number of hours or at least a more narrow, ideal range for your tank will depend on the other factors we explain next.

What type of environment are the fish/plants accustomed to?

Do you have or are you looking to start a freshwater or saltwater aquarium? Do you want tropical plants? Fish that can survive in brackish water? Plants and fish from different parts of the world need varying amounts of light. And you’ll want your tank to mimic the natural environment of your plants and fish as closely as possible.

For a tropical aquarium and fish that come from shallow waters, we recommend shooting for a photoperiod between 10-16 hours. For fish that come from deeper waters further from the equator, we recommend 6-8 hours.

Note: If you bought tropical fish that in the wild are usually exposed to a lot of light, but were grown in an aquarium and are accustomed to less light, then less light will be better for those particular fish.

When looking at the photoperiod, we need to consider every light source the tank is exposed to. If your aquarium light turns off for the day after 8 hours but it’s next to a window with sun coming through it, that will extend the photoperiod and throw off the balance of the aquarium ecosystem. This could lead to negative outcomes such as increased algae growth. You’ll want to either include the natural light exposure as part of the calculated photoperiod, or move your aquarium to a more controlled lighting environment. 

What Aquarium Lighting Method Will You Use?

Most commonly, aquascape enthusiasts will apply the continuous lighting method. Using this method, the photoperiod is determined and the light source is turned on for the entirety of said period. Using the siesta method, the light source is turned off for a period of time in the middle of the photoperiod. Much like an afternoon siesta in the middle of the day in Spain, the light source takes a midday break. The argument for the merits of this method have to do with the balance of nutrients in the aquarium, namely, CO2. 

During the photoperiod, the plants are consuming light, CO2, and other nutrients in photosynthesis. The siesta method allows a break in the middle of the photoperiod where photosynthesis is paused and CO2 levels are allowed to build up. When the light source is turned back on there are heightened levels of CO2 to accommodate photosynthesis.

Parting Thoughts

After considering the age of your aquarium, the natural environment of your friends calling it a home, and the lighting method, you are able to determine how long to leave an aquarium light on. Finding the right photoperiod length is so important for achieving and maintaining a balance in the ecosystem. Starting and ending that photoperiod consistently each day is just as important. 

Having the right light with the best intensity and color temperature is also important. But that’s an article for another day. If you’re looking for a quality aquarium light that has it all and doesn’t break the bank, we recommend the NICREW Aqualux Plants LED. It comes with a built-in timer and totally customizable color temperature and intensity settings. It’s great to see how far the industry has come in just the last few years. NICREW continues to put out great aquarium products at affordable prices. Check it out on Amazon with our affiliate link below.


Can I leave aquarium lights on 24/7?

You should not leave aquarium lights on 24/7 as it will lead to an unhealthy tank with distressed fish and excessive algae growth. Instead, only leave the lights on for the appropriate length of time for your tank, typically around 8 hours a day. You can use a timer to help you maintain an adequate and consistent photoperiod. 

Does too much light cause Algae?

Too much light, or too long a photoperiod, can cause algae growth in your aquarium. To avoid this issue, make sure the photoperiod is not more than the recommended amount for your type of aquarium plants and fish. You’ll also need a photoperiod that balances with the amount of nutrients (CO2 and minerals such as calcium and phosphorus).