Should I Use Blue Light In My Fish Tank?

by | Articles, Blog, Freshwater Fish, Saltwater Fish

Blue lights in fish tanks are everywhere these days. LEDs changed a lot in the aquarium hobby when it comes to lighting, and blue light LEDs come with lots of questions: Can your fish sleep if you have them on? Will they harm your fish? Let’s dive in and find out.


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1 What does blue light in a fish tank do?

Blue light is a fun, aesthetically pleasing low light popular for use in aquariums for displaying fish at night. It’s often used to show off glofish and corals. Importantly, blue light does not have a specific, beneficial function for fish. Rather, it’s something that humans created as a cool way to view and display fish.

You’ll also hear blue light referred to as moonlight, though this is factually incorrect, as blue lights (and lights advertised as “moonlights”) are on the visible light spectrum and are still actinic lights. This means that plants will still be photosynthesizing and algae is still able to grow. It’s not true moonlight.

This is why experienced hobbyists recommend only using them for short term ambience. Don’t leave them on all night. Because fish don’t have eyelids, they rely on darkness in order to get some rest. If you leave lights on all night, your fish won’t be able to sleep. Plus, plants and corals will still be photosynthesizing, and excess light causes the dreaded algae outbreak.

As one commenter put it, “These lights are for you, not your fish.”

Plus, light affects fish’s bodily rhythms, not so unlike humans. Fish rely on cues from their natural environment in order to function; Fish endocrine systems, reproductive systems, hormones, and more all take their cues from the fish’s environment.

The light tells the fish’s system to stay awake, and to be on alert for predators. While fish tend to sleep intermittently (10 minutes here, 20 minutes there) they do rely on darkness as the time during which they can rest, sleep, and not be hunted.

Light affects cortisol levels in fish, and keeping it on all the time prevents fish from sleeping well, also generating stress hormones that they would not otherwise be subject to in the wild.
If you’re an aquarist, you know that causing stress to fish is bad news. Stress inhibits fish’s immune systems, makes them susceptible to illness and disease, and can shorten their lifespan.

Blue lights can be okay for some nocturnal fish as long as it’s used short term and you keep it on a timer (e.g., you don’t leave it on the whole night).

Leaving blue light on all night is particularly damaging to fry and young fish, as studies have shown that their eyes may not develop properly as a result.

Tropical Fish Under Blue Light

2 Is blue light good for aquariums? When should you use blue light?

Blue light is best used as a transition lighting between day and night. It’s common for hobbyists to turn on the blue light between 7 and 10 PM, for example, and then turn the light off for the night. Similarly, you can do the same in the early morning before turning on the regular lights for the day.

The general idea is to mimic natural light in your tank as best as possible: Darkness at night, gently increasing light for ~1h at dawn, then regular daylight, and then gently decreasing late for ~1h at dusk before turning off the lights completely.

Some tanks come with built in sunrise/sunset features with their lighting. You can use timers to automate this as well.

Fish, invertebrates, and corals are significantly affected by lunar cycles. Some aquarists use blue light to mimic moonlight, and this can work as long as you follow the actual schedule of the moon phases (rather than just leaving the light on all night).

A good rule of thumb is that anytime you can see and discern what’s inside your tank, there is enough light to be interfering or sending cues to your fish’s system.

Some people will have a very dim blue light that only illuminates the very top part of the tank, leaving the rest in darkness. This is a more suitable solution for fish, but be aware that if plants are getting that light, you can still be feeding algae.

A good guideline is to never leave any light on in a fish tank for more than 12 hours per day.

Some exceptions to this are biotope tanks that have tons of places where fish can escape light completely, and shrimp and snails, which are less affected by light cycles.

Each tank requires its own individual, specialized lighting, based on the plants, plant, configuration, and fish species in the aquarium. When establishing a tank, it’s a good idea to ramp up lighting intensity and frequency slowly, so that you can find the right lighting for your tank configuration. Keep a log with dates and lighting information so that you can track how your plants and fish are doing according to any changes you make.

Just remember that blue LED light can cause algae problems because it’s still actinic light, and too much light of any kind can cause algae to take over your tank quickly. (In fact, a common method to solve algae problems is light deprivation.) I can tell you right now, I strongly recommend against leaving a light on all night in your tank, both for the sake of your fish, and for the sake of preventing algae.

It’s worth noting that different lights and brands vary in terms of how much actual blue light is coming through. It might appear blue to the eye, but still have significant amounts of white and red in it.

Also important to note: Blue lights are excellent for corals! In fact, the best reef tank lights that money can buy contain a significant amount of blue spectrum lighting as a result. Many reefers tinker with the color ratios of their lights to find the optimum balance for their tank (for example, 50% white, 50% blue, or 80% blue, 50% white, and so on).

Check out this deep dive for more info:

3 Do you need a blue light in a fish tank?

No, blue light is definitely not necessary in a fish tank. It’s more of a fun aesthetic touch to use intermittently when you want to show off your tank in the evening without having a bright light on. It’s also fun for displaying glofish. But these blue lights were developed for our sake, not for the fish; it’s not beneficial for the fish and it’s definitely not a necessity in a tank.

4 Can fish sleep in blue light?

No, fish cannot sleep in blue light. Fish don’t have eyelids, so they rely on darkness in order to get some sleep. If a blue light is on, their eyes are still open and they won’t be able to sleep. Light also sends cues to fish’s endocrine systems, causing them to be more awake, and can contribute to a buildup of stress hormones over time.

5 Can blue light harm my fish?

Coral reef under blue lights

Anecdotally, the short answer is no. The long answer is clear as mud. There have been several studies performed, the results of which are all over the map when it comes to whether or not blue light can be harmful to fish. One study showed retinal damage to goldfish and zebra danios after blue light exposure (Effects of blue light spectra on retinal stress and damage in goldfish, Song, Jin Ah)… Another showed that baby guppies developed better in blue light than in green or red light (Influence of Colored Light on Growth rate of Juveniles of Fish, Ruching, A.B.)

Because of the ambiguity, I prefer to play it safe and stick to natural light cycles, using blue light minimally and in moderation. Personally, I would not use blue light for young fry and developing fish.

6 When should I turn off blue light in my fish tank?

You should turn off blue light in your fish tank when you go to bed. I recommend using blue LED light primarily as a transition light (dusk, dawn) and for short-term display purposes in the evening. Leaving a blue light on all night can contribute to allergy problems and algae blooms, will prevent fish from sleeping, and can contribute to increased fish stress.

7 Conclusion

Blue LED lights are for us, not the fish. But they’re fun and very aesthetically pleasing when used in moderation. I recommend using them as a way to replicate the natural cycles of daylight, turning your blue lights on during dusk and dawn. It’s a nice way for your fish to start and end the day, too.



To say that I’m obsessed with all things saltwater is a bit of an understatement. Aquarium Passion has served freshwater and saltwater aquarium hobbyists for over 10 years, and I'm committed to keeping the information accurate and free. My post-bac certification in Sustainability informs my writing about aquarium conservation efforts. When I'm not writing at AP, I'm out in the ocean or researching weird fish.


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