White and blue lights on Radion XR 15

Can I leave blue light on in a fish tank?

A lot of popular LED light models have white and blue lights. Blue light is an important wavelength for photosynthesis (the process where plants turn light into sugar/energy).

Blue light is also frequently used to simulate dawn and dusk light cycles. For example, in an 8-hour photoperiod, an aquarist may choose to run a light with White and Blue LEDs as follows:

  1. 12 PM to 2 PM: Blue LEDs only
  2. 2 PM to 6 PM: Blue + White LEDs together
  3. 6 PM to 8 PM: Blue LEDs only
  4. 8 PM until 12:00 PM next day: all LEDs OFF
LED Aquarium lights

But the existence of the Blue LEDs, often set up on a different switch setting, as well as the tendency to use those lights in the evening, creates a question I hear fairly commonly.

Can I leave the blue light on in a fish tank?

Blue light in a fish tank should not be left on for more than 12 hours each day. No light in a fish tank should be left on more than 12 hours each day. The fish, invertebrates, and even photosynthetic plants or corals in your tank will be healthiest and grow best with 12-16 hours of darkness each day. If you occassionally forget to turn the blue light off in a fish tank, that is okay, just turn it off next time, or set it on a timer or smart plug, so that you don’t have to remember.

Is blue light bad for my fish?

It seems highly unlikely that normal blue light with typical aquarium use is bad for your fish. There are a few scholarly publications on this and related topics, but the findings across studies seem a bit confounding, at best.

For example, a couple of studies concluded that blue light damages fish eye retinas. This was studied in goldfish and zebrafish (called zebra danios in pet stores near me). They did demonstrate and publish results that showed blue light exposure increased signs of stress and damage to the cells in the retina.

But there are other studies that show blue light actually reduced the stress response in Tilapia, which is a food fish, but is also a Cichlid.

Another study showed that the development of baby guppies was stronger/faster/better in blue light than it was in green or red light.

A different study showed that the species of fish they studied preferentially swam to the areas illuminated with blue or green light, but swam away from yellow and red light.

Why would fish have an instict to swim towards a light that is bad for them.

Here is a table summarizing the studies involving blue light and its effect on fish

Blue light and fish summary conclusionStudy titlePublicationLead Author
Blue light reduced stress response in tilapia.Environmental blue light prevents stress in the Nile tilapiaNeurosciences and behaviorVolpato, G.L.
The impact of the color of light on the growth of juvenile fish varied by species. Notably, guppies developed better with blue light. Red light decreased growth in all species.Influence of Colored Light on Growth rate of Juveniles of FishFish Physiology and BiochemistryRuching, A.B.
After 1 week of exposure to blue light from an LED, signs of stress were elevated as well as evidence of damage to retina cells (likely leading to cell death) in goldfish.Effects of blue light spectra on retinal stress and damage in goldfishFish Physiology and BiochemistrySong, Jin Ah
A species of fish in Tibet swam towards green and blue lights but away from red or yellow, suggesting an affinity/preference for blue and green.A detailed analysis of the effect of different environmental factors on fish phototactic behavior: directional fish guiding and expelling techniqueBehavioral Ecology of Aquatic AnimalsXu, Jiawei
Rearing goldfish under blue light improves their growth rate, immune responses to stress, and behaviorEffect of different monochromatic LED light colors on growth performance, behavior, immune-physiological responses of goldfish Carassius auratusAquacultureNoureldin, Salwa M.
Summary of recent publications about the effects of blue light on fish

Blue light has also been demonstrated to be extremly important to the growth of corals. Most of the facilities that grow corals commercially use a light spectrum combination that is heavily skewed towards the blue light part of the spectrum called AB+ that has been shown to produce the best growth in corals. The top of the line reef aquarium lighting manufacturers, like the Radion XR 15 BLUE lights above my tank, provide templates to automatically use this spectrum and are specifically designed with EXTRA blue LED diodes.

White light becomes more blue the deeper underwater you go

Finally, if you have spent any time snorkeling, or in a deep pool, you noticed that the lower you go, the bluer the light gets. Now, I know not every fish comes from a blue ocean environment, they may spend their time on the surface, where the light would be more white. Also, many freshwater species are in yellow/brown/turbid waters. So perhaps they would be more impacted, but I’m not sure that study has been performed.

One final note–zebrafish are used in studies often as a developmental biology model. My hunch, suspicion, hypothesis is that the studies showing blue light damage to fish eyes were intended more to inform understanding the impact of the blue light from electronics on our eyes than they were intended to show that you shouldn’t leave a blue light on over your fish.

Will plants grow better if I leave the lights on?

Plants and other photosynthetic organisms grow best when they have both light and dark periods. During the light period, plants and photosynthetic organisms like zooxanthellae harness the energy in light to make sugar with carbon dioxide and water. During the dark period, these photosynthetic organisms are like any other organism, where they need to consume those sugars for energy. Blasting too much light, out of balance, disrupts the dark phase and results in less vigor, not more.

That is why it is important to establish a consistent and balanced photoperiod for your tanks, or when trying to grow photosynthetic organisms like phytoplankton. A photoperiod of 8 to 12 hours on and 12 to 16 hours off is typically ideal.

White and blue lights on Radion XR 15

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

Fish, coral, and other invertebrate spawnings are often tied to lunar cycles. So the notion of recreating moonlight in your fish tank, instead of just total darkness may encourage more natural cyclical behaviors. But blue lights should not be left on in a fish tank for longer than a few hours, to simulate moonlight.

I am currently running the Corallab AB+ template on Radion XR 15 Blue LED lights, and the program is set for a gentle 3 hour moonlight period, which also creates another interesting viewing opportunity. I enjoy watching the fish and corals start to get ready for the night as the moonlight program gets closer to ending.

Blue lights simulating lunar cycle
This blue light and white light are scheduled to run only 3 hours each night

You could also DIY your own moonlight by having a dim blue light, white light, or combination light mounted above the fish tank. If your LEDs are dimmable, you could set them up to brighten and dim in conjunction with a 28 day cycle to recreate the phases of the moon.

Roxanne, you don’ t have to put on the red light

I just like that song. Didn’t understand it until I got older :). I just counted, and that song repeats “put on the red light” 25 times. Yowza!

Sting, ironically illuminated with a red light. I guess Roxanne did have to turn it on.

Related articles

Check out these other related articles:

Growing Phytoplankton at home

Best Reef Aquarium Lights


Noureldin, Salwa M., et al. “Effect of different monochromatic LED light colors on growth performance, behavior, immune-physiological responses of gold fish, Carassius auratus.” Aquaculture 538 (2021): 736532.

Ruchin, A. B. “Influence of colored light on growth rate of juveniles of fish.” Fish physiology and Biochemistry 30.2 (2004): 175-178.

Song, Jin Ah, and Cheol Young Choi. “Effects of blue light spectra on retinal stress and damage in goldfish (Carassius auratus).” Fish physiology and biochemistry 45.1 (2019): 391-400.

Volpato, Gilson Luiz, and R. E. Barreto. “Environmental blue light prevents stress in the fish Nile tilapia.” Brazilian Journal of Medical and Biological Research 34 (2001): 1041-1045.

Xu, Jiawei, et al. “A Detailed Analysis of the Effect of Different Environmental Factors on Fish Phototactic Behavior: Directional Fish Guiding and Expelling Technique.” Animals12.3 (2022): 240



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