Can I Leave Blue Light on in Fish Tank?

One of the pleasures of owning freshwater aquariums is creating a natural looking environment complete with a variety glowing lights and incandescent bulbs. It helps bring the fish tank to life! Some of the best-looking fish tanks use incandescent bulbs with different sets of colors, which help bring out the natural colors of certain fish species and aquatic plants. Blue lighting is commonly used by aquarists for various reason such as simulating moonlight effects or as a fish tank night light.  

But you may be surprised to know that not all lights are created equal and the duration of light including intensity and spread has a significant effect on the overall health of your aquarium.

As an aquarist it’s our responsibility to make sure that we keep our fish healthy while enhancing the appearance of the fish tank. Proper lighting is essential for fish tanks because lighting helps drive the process of photosynthesis in aquatic plants and blue lighting specifically can be beneficial for different fish species, aquatic plants, and corals. In this article, we will dive into the effects of blue lights and answer some commonly asked questions about blue aquarium lights.

Can I leave blue light on in a fish tank?

The quick answer is no. As a general rule, all light fixtures including blue lights should be turned off at night time. The maximum number of hours to leave aquarium lighting on is 12 hours. After that you should cycle your lights off for at least 12-16 hours, especially during night time. Doing so allows your fish to adapt to a natural light cycle that would be similar to their own natural environments.

You can follow a simple schedule like this one: leave aquarium lights, like white light or blue light, on during the day, then turn them off at night time. Just remember that, like humans, many fish species follow the same day and night cycles. Lighting is important not only for plants that depend on light to make their own food, but also for aquarium fish in order to manage their daily lives. Too much light can cause fish to stress and make it harder to fight off diseases and can cause excessive algae growth.

Blue Light in Fish Tank
Blue Light in Fish Tank

What does blue light in fish tank do?

We already discussed how lighting in a fish tank is mainly used by plants and fuels plant growth which in turn helps keep a healthy fish tank. But lighting, in combination with other factors such as water temperature, can also affect fish and their growth rates, metabolic rates, stress levels and reproductive cycles. Depending on the fish species, different lights from across the color spectrum can have different effects.

Blue lighting is mainly used by an aquarist to help them see the fish tank at night. It can offer aesthetically pleasing ambiance and bring out the colors of certain aquatic plants. Blue lighting is great as an aquarium night light for that reason. There are other uses for blue light however. Some research has shown that blue light can help eliminate bad bacteria in the water. It can also help reduce stress for certain fish species which helps minimize the chances of diseases. Blue lighting helps minimize excessive algae growth, which can contribute to an unhealthy fish tank.

Green light, red light and white light also play different roles in a fish tank. More research is needed to help determine if green light can actually be useful for plant growth but there are some studies that have shown green light may help increase growth rates in juvenile fish.

Red light may increase the rate of hunger and therefore behavior of certain fish species. Some red light, however, has also been shown to decrease growth rates in some juvenile fish species.   

White light is best for mimicking the daylight hours and therefore help regulate circadian rhythms for many fish species. Overall, it is always helpful to remember that light is mostly used to fuel aquarium plant growth, and aesthetic lighting in other instances.

Is blue light good for fish?

Yes, blue light can be good for fish if it is used in the correct way. The correct way to use blue lighting is to recreate the natural environment of your fish. That means leave it off, or at low brightness, during the day and turn it off at night, or limit it for just a few hours. That way it mimics the moonlight and helps regulate the circadian rhythm of your fish. Fish need total darkness (about 8-12 hours depending on the fish species) each night.

Blue light can be harmful to aquarium fish indirectly by stimulating excessive algae growth which can possibly harm many fish species. Some plants or corals may use blue light to help with growth, but more research is needed. In general, blue light can be safe to use if managed correctly.

Here’s a chart that shows the optimal hours of light depending on the type of fish tank you have:

Average Aquarium10 hours of light
Tropical fish and plants12 hours of light
Cold water fish and plants8 hours of light
No plantsMinimal light needed
Blue Lighting in Fish Tank
Blue Lighting in Fish Tank

Do blue aquarium lights cause algae?

Blue lighting can cause excessive algae growth if it’s left on for too long. What matters the most is the duration and intensity of light throughout the life of the tank. Too much light from any source and any color can cause excessive algae growth. One way to reverse this is by reducing the time light is left on. For example, light fixtures can be set to turn off or you can manually turn off lights after 8 hours, if algae is a problem in your tank.

Finding the right combination of aquarium lighting will be easier if you monitor the levels of algae growth in your tank. It is also known that direct sunlight can be the cause of too much algae in a fish tank. The right combination of florescent lighting, including blue light, and natural sunlight will help provide the optimal growth environment in your fish tank. 

Does blue light help aquarium plants grow?

All plants, including aquarium plants, need light to convert carbon dioxide and water into the energy they need to live. They can get this light from many different sources including ambient lighting, light fixtures, direct light, or florescent lights, etc. Blue light can be just as helpful as white light for example as a source of light, but the key is balance. Blue lighting also has other effects such as minimizing the levels of pathogens in freshwater and saltwater habitats which may indirectly be beneficial for aquarium plants by promoting a healthy environment.

It is best to mimic natural conditions and provide your fish tank with adequate lighting during the daytime and blue light can be used in addition to other lights like white lighting. However, it is important to have dark periods otherwise you risk having too much light enter the tank which can disrupt a healthy environment. Aim to make each photoperiod, or the light cycle, be 8-12 hours on and 12-16 hours off.  

You can mix and match different types of light such a red light, green light, or white light during the day and in the evening, turn off all your florescent lights. One way you can get different sets of lights or blue aquarium lights is by purchasing florescent tube lighting. The most common light fixtures include fluorescent lights which are capable of changing different colors depending on the bulb you purchase. Always check the manufacturers packaging for the most accurate information. Florescent lights typically need to be changed every few years. Metal halides are often used by aquarists even though they can be more expensive. Metal halides are great for reef tanks for example because they have a better light spread and provide more light coverage throughout the fish tank. These types of lighting can work great separately or in combination, but always be aware of making sure you are not extending the photoperiod or providing too much light.

Can I leave blue light on in a fish tank to simulate moonlight?

Having blue lights on all night in order to simulate moonlight can be damaging to your fish. Like us, fish also need periods of complete darkness in order to sleep. If there is too much bright light and it is left on continuously, this can have really bad effects on the environment in your fish tank as well as stress out your fish. Up to 16-24 hours of non-stop lighting is bad and should be avoided.

Some species of aquatic life, such as corals or invertebrate spawning, use the lunar cycle to manage their reproductive cycles or other life processes. If you intend to leave light on at night, it is best to create a separate fish tank for certain fish or invertebrate species that can survive on longer exposure to blue light.

As a general rule, blue lighting should be left on for only a few hours to mimic moonlight and then be turned off.

Blue Light in Aquarium
Blue Light in Aquarium

Does aquarium need light at night?

As a general rule, most aquariums need some type of light source, and the amount of light depends on the fish and plant species you intend to host. A well planted community tank may need more light than a fish tank that houses just one particular fish species. Light becomes most important when you intend to have aquatic plants versus if you intend to just have fish. Lighting also makes it easier for you to see and enjoy your fish tank and can help illuminate the tank when providing tank maintenance. Often, the natural space your tank is in will provide enough ambient lighting to adequately host fish species.

As mentioned earlier in this article, plants need sufficient lighting to do their process of photosynthesis. But if you have too much light than you risk causing other problems such as excessive algae growth. Always think about the native environment your plants and fish typically live in. Some cold water species of fish like zebra fish and minnows come from temperate climactic zones and experience varying levels of sunlight. Tropical fish live in native habitats where they often receive over 12 hours of daylight consistently. Research the native environment your fish come from and do your best to mimic those same conditions in your fish tank.

Consider that some species, such as nocturnal fish, thrive on less light. Cichlids and tetras for example do best when there is minimal lighting. Nocturnal fish live most of their lives in the cover of darkness and regulate certain physiological and metabolic processes by the changing phases of the moon. This means that providing some type of moonlight, like blue led lights, can be beneficial for these organisms.

Here’s a list of aquatic life that may do well with blue light mimicking moonlight environments (Not: it is still important to provide completely dark photoperiods even with some nocturnal species):

Moray eels
Many invertebrates


As a general rule, you should never leave blue light on for longer than 12 hours at a time, but an even better range is 8-10 hours of light followed by total darkness. Blue light has many uses but is primarily used for promoting plant growth and illuminating your fish tank for useful and aesthetic purposes. Blue light can be good for fish indirectly by helping to reduce bacteria and other pathogens in the water, and by proving at least a moderate source of light energy for aquatic plants. Blue light can promote algae growth so it is always important to monitor your tank for signs of too much lighting. Blue light in combination with other lights such as white light, can fuel plant growth. Never leave your blue lights on during the night to try and mimic moonlight. If you want to provide some ambience in the form of moonlight using your blue led light, you should do so for only a few hours, then provide your fish tank with total darkness. Aquariums need varying levels of light, although the ambient lighting in your space may be adequate enough for a healthy fish tank. Do your research and check with species of fish and plants you intend to purchase and make lighting adjustments and purchases bashed on those fish and plant species. Some nocturnal species thrive on less light. Always try to mimic the native habitats of fish species for the best chances of having a successful and enjoyable fish tank.