Is Blue Light Good For Corals? (Common Mistakes To Avoid)

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We all know that blue light is a common practice in reef tanks. Some aquarists even claim it makes their corals glow brightly.

But how do corals actually react to blue light? Is it good to use this color in a coral tank?

These are the questions I constantly asked myself in my early days of fishkeeping. So, to make life easier for you, I decided to dedicate an entire article to this topic.

Let’s dive right into it.

Corals under blue light in a reef tank I once had.

Is Blue Light Good For Corals?

Blue light is beneficial for corals as it encourages zooxanthellae growth without changing water temperature, which is vital for their survival. Corals have also adapted to photosynthesize with the help of blue light, especially the species growing in deep water.

Light is good for reef tanks. It allows corals to thrive, giving these organisms access to sufficient oxygen.[1]

If you prefer to keep your aquarium away from direct sunlight to prevent algae growth, you may turn to white LEDs that mimic the sun.

But what about blue light? You might be surprised to learn that blue light is more beneficial to corals than white light. Consider the following:

1. Blue Light Grows Algae That Are Vital To Corals

The corals you encounter in most tanks are photosynthetic. This makes them reliant on sunlight.

Corals have a symbiotic relationship with zooxanthellae, a type of algae that grows on their branches.

Corals feed on the sugar the algae make when you expose them to light. In return for that nourishment, corals provide zooxanthellae with carbon dioxide.

They also protect the algae. Therefore, you cannot raise healthy corals in a tank without adequate lighting.

That doesn’t mean you should overwhelm them with light. In the wild, they live in sunny locations with warm water.

But hot water will harm them by compelling the algae they host to produce toxins.

The corals respond by ejecting the algae. This is bad for them because they need the algae. The result is coral bleaching.[2]

Fortunately, blue light encourages algae growth and does not change the water temperature. This fulfills two vital needs for corals.

Corals and zooxanthellae live in symbiosis.

2. Corals Are Used To Blue Light

If corals do well in sunlight in the wild, you might wonder why anyone uses blue light when your reef tank can clearly succeed in the presence of white daylight LEDs.

The answer is simple. Sunlight is not purely white or yellow. It has a broad spectrum of colors.

Most aquarium corals come from deep waters in the wild. When sunlight hits those deep waters, it filters out the yellow, orange, green, and red.

Blue is the only significant color that reaches the corals at the lower depths. 

As a result, many coral types have adapted to these conditions, learning to photosynthesize with the help of blue light, especially the species growing on the ocean floor.

Some aquarists use a combination of blue and purple LEDs.[3] The reason for this is that shades of purple also reach the depths of the ocean.

3. Corals Use Blue Light To Build Their Skeletons

Some aquarists use kelvin ratings to select aquarium lighting for their reef tanks. Kelvin ratings look at the color temperature.

Some newcomers confuse this unit with a light’s intensity, but the two are not connected.

You can use the kelvin rating to identify the color. Lights with lower kelvin ratings have warmer colors.

This is where you find yellows and red. The higher the K number, the bluer a light becomes.

For instance, a 6,500K bulb will give you more light than its 20,000K counterpart because 20,000K lighting is very blue.

You don’t need bright or intense lighting if blue is your chosen color. You can get away with less intense lighting. Photosynthesis will still occur.

All in all, blue is best for corals. It stimulates calcification, a process corals use to create calcium-rich skeletons.

How Long Do Corals Need Blue Light?

Corals are not the most complicated organisms in the world. However, their lighting needs require more thought than what you encounter with fish. For instance:

1. The Coral Type In Your Reef Tank

Corals don’t have the same lighting demands. Some of them require intense lighting. One example is SPS (small-polyp scleractinian) which needs a PAR of 200 to 400.

You also have types that thrive in low-intensity lighting environments, such as soft corals (PAR of 25 to 125).

LPS corals fall somewhere in the middle. They need standard intensity (PAR of 50 to 200) to photosynthesize. The coral type will influence your decision.

Euphyllia, a type of LPS coral, requires standard lighting intensity.

2. The Lighting Type You Have

If your tank is new, you can install a lighting type that meets your coral’s needs.

But if you installed the lights before adding the corals, you should adjust the lighting schedule to fit the lighting type’s strengths and weaknesses.

Your biggest concern is the metal halides because they are very high-intensity lights.

Admittedly, any light can become a high or low-intensity light if you make the proper adjustments. The higher the intensity, the shorter the duration.

Limit the lighting period to six hours at maximum intensity (all bulbs on). Otherwise, stick to the usual 8-12-hour lighting pattern.

Keep in mind that you have fish in the tank. You can’t expose them to 24 hours of blue light.

Don’t exceed 12 hours. The tank’s inhabitants require a period of darkness during which they rest.

Keeping the blue light on won’t let them sleep properly. The same goes for corals, which have their own circadian rhythm.

Don’t assume that you can improve the coral’s health by extending their exposure to light simply because they use light to grow.

A study from Cambridge University (Miriam Schutter, Ronald Osinga, Johan A.J. Verreth, Max Janse, Rosa M. van der Ven) investigated the matter and found that increasing exposure to light did not boost coral growth.[4]

Instead, continuous lighting led to coral bleaching. The corals in question died after two weeks.

Just like humans, aquatic animals require several hours of darkness to sleep.

3. The General Lighting Schedule For Corals

If you have a conventional reef tank that houses corals and fish, and you installed dimmable LEDs, your lighting schedule will look something like this:

10 amLight on (20 percent)
12 amIncrease light to 50 percent
1 pmIncrease light to 75 percent
2 pm – 8 pmLight is at full (100 percent) intensity
9 pmDecrease intensity to 60 percent
10 pmDecrease intensity to 20 percent
11 pmShut lights off

Your schedule will vary depending on when you wake up and go to sleep.

For instance, if you leave home at 8 am, you may activate the lights at 7 am or whenever the sun comes up.

Additionally, you don’t have to adjust the light’s intensity. The goal is to mimic conditions in the wild where the sun slowly rises and sets.

But you can turn the lights on at full intensity in the morning and switch them off at night before you go to sleep.

4. The Recommended Blue Light Schedule For Corals

Blue lights will change your schedule, but only slightly.

Aquarists use daylight bulbs during the day. At dusk, instead of lowering the intensity of the daylight bulbs, they switch to blue lights.

The blue LEDs stay on for the next five or six hours, only going off at 11 pm or midnight when the aquarist goes to sleep.

At dawn, they switch the blue lights on once more to mark the transition between night and day. You can even switch from blue light to daylight bulbs at full intensity. 

Again, the time you choose to turn the bulbs on or off will depend on when the sun rises and sets in your area.

You can make any schedule you want, especially if you keep the aquarium in a dark room where it can’t access natural light.

Excellent YouTube video explaining the ideal reef tank lighting schedule

Does Blue Light Make Corals Glow?

Yes, it does. 

Jorg Wiedenmann (University of Southampton, UK) worked with researchers from the Interuniversity Institute for Marine Sciences in Israel to investigate mesophotic reefs 100 to 330 feet deep.[5]

These corals glowed red and green when Wiedenmann exposed them to blue (or UV) light. And this is not a unique response.

Corals in shallow water have fluorescent proteins that allow them to glow. This defends the organisms from the harmful effects of the sun’s rays.[6]

Can You Run A Reef Tank With Just Blue Light?

You can maintain a reef tank with just blue light because it is the most beneficial color.

This is in contrast to red light, which in some cases can harm corals. You can also expose your corals to green light, although this is not necessary as well.

If you have fish, they will appreciate the blue as it is a soothing color. It has a calming effect. Just remember to turn the lights off during the night.

Pro Tip: Feeling uncertain about how to light your coral tank? Don’t worry, I’ve got you covered with my ultimate guide on the topic.


To cut things short, these are the main key points about coral and blue light:

  • Blue light encourages the growth of algae that live in symbiosis with corals.
  • Corals can perform photosynthesis with blue light, just like aquarium plants.
  • The intensity of blue light is not important for photosynthesis to occur in corals.
  • Don’t exceed 12 hours of blue light and give the tank’s inhabitants a period of darkness for rest.
  • Continuous exposure to light does not necessarily improve coral growth and can lead to coral bleaching.