Is Red Light Good For Corals? Do They Actually Need It?

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We all know that aquarium corals do great with blue lights. However, less is known about red light and its role in coral growth.

There is a common debate claiming that red light contributes to the photosynthesis of algae, and is therefore essential for corals as well.

However, some researchers argue just the opposite. So, to save you a lot of time and trouble, I decided to devote an entire article to this topic.

Let’s dive right into it.

Is Red Light Good For Corals?

Red light can be beneficial for corals as it can be used by zooxanthellae for photosynthesis, but some studies have found that excess red light can harm corals, and in nature, red light does not reach corals at depths where they typically grow.

Technically, red light can be good for corals, although it is not that necessary. You have a lot of information to support this claim:

1. Zooxanthellae Don’t Need Red Light For Photosynthesis

Zooxanthellae are photosynthetic organisms that live with corals. Corals and zooxanthellae have a symbiotic relationship.

Zooxanthellae provide nutritional resources (glucose, glycerol, amino acids) that corals use to make proteins, fats, and carbohydrates.^[1]

In other words, the health of the zooxanthellae is vital to a coral’s well-being. Zooxanthellae matter to this discussion because they use light to photosynthesize.

When an aquarist asks whether red light is good for corals, they want to know what it does to the zooxanthellae.

Without these organisms, sunlight wouldn’t matter as much to corals. After all, corals are animals.^[2] They don’t photosynthesize.

Depriving zooxanthella of the appropriate nutrients and conditions will kill the corals in the long run because they can’t survive long without these organisms.^[3]

However, Zooxanthellae are known to use mainly white and blue colors for photosynthesis. Little evidence was found about the red light.

**Zooxanthellae and corals live in symbiosis.**

2. Red Light Doesn’t Reach Corals In Nature

Have you ever wondered why red fish are so difficult to see at depths of 100 meters or more? Sometimes, they appear black.^[4]

In other cases, you can’t see them at all. Sunlight consists of a wide range of colors. But when it hits a water body like the ocean, it reflects colors with long wavelengths.

A paper from Wageningen University (Ronald Osinga, Max Janse, Marcel Janssen) noted that blue light has a shorter wavelength, which is why you can see it at 100m or below.^[5]

On the other hand, red light disappears after a while (between 1m and 5m). This explains the blue tinge you see when you go diving.

The bottom of a water body is blue because blue light is present, reflecting back to your eyes.

The depth argument doesn’t matter to tanks because they are too shallow for the light from a red LED to attenuate significantly.

However, it explains the preference corals have for blue light.

Experts are convinced that corals at the lowest depths of water bodies adapted to photosynthesize from blue light because the other colors were absent.

**A beautiful coral illuminated with red light.**

3. Some Studies Have Found That Red Light Can Harm Corals

This is where the confusion arises. Photosynthesis can occur in red light.^[6] Blue light is better. However, red is a decent alternative.

Zooxanthellae have absorption pigments (chlorophyll in A, B, and C forms and carotenoid absorption pigments such as peridinin and beta-carotene) that respond positively to light in the blue, violet, and red ranges.^[7]

This is encouraging because it shows that zooxanthellae can turn light energy into chemical energy using red LEDs.

But if that is true, why are some aquarists hesitant to embrace red? Well, some studies have identified red as a threat to corals.

A prominent example is a 2014 paper in PLoS One (Tim Wijgerde, Ronald Osinga, Luc Vogels, Miguel C. Leal, Anne van Melis, Claudia Mutter, Catarina I. F. Silva).^[8]

The study found that red light represses chlorophyll A synthesis. Necrosis was observed in corals grown under red and blue red light.

The research team theorized that the genotype in the study may have originated from depths where red light is absent.

As such, these corals have a heightened sensitivity to excess red light. However, at the moment, that is more of a theory than a proven fact.

Studies like this one have encouraged aquarists to approach red lights cautiously.

But then Jorg Wiedenmann (Coral Reef Scientist) from the University of Southampton has noted in previous studies that corals turn blue light into orange-red lights.^[9]

Even though blue light is dominant at lower depths, it doesn’t penetrate coral tissue as effectively, which is problematic because zooxanthellae occupy positions deep inside the coral.

Orange-red lights can’t reach the lowest depths of the ocean, but they can penetrate the coral’s tissue better than blue light, allowing photosynthesis in zooxanthellae to occur.

This suggests that red light is beneficial to corals, however, it is still far from conclusive.

**In nature, red light does not penetrate deep enough to reach corals.**

Will Using Red Light For Corals Cause Algae?

Algae are photosynthetic. Therefore, people expect red and blue light to cause algae growth, and they are not wrong.

However, don’t expect an algae infestation to occur simply because you added red lights.

It takes a combination of factors for algae to run amok, including poor-quality water, a nutrient imbalance, inefficient filtration systems, etc.

If the algae are running wild in your tank, something else is to blame, not the red LEDs.

How Long Do Corals Need Red Light?

A conventional reef tank requires two to three hours of light at the start, during which you slowly ramp up the intensity.

After that comes eight hours of full intensity, and two to three hours at the end of the day, where you gradually lower the intensity.

In other words, your schedule will look something like this:

9 am	Lights go on at 20 percent
10 am	Increase intensity to 50 percent
11 am	Increase intensity to 75 percent
12 am – 8 pm	Full intensity
9 pm	Decrease intensity to 60 percent
10 pm	Decrease intensity to 20 percent
11 pm	Lights off

This pattern mimics what corals get from the sun in nature.

Use white daylight bulbs during the day. Technically, white daylight bulbs include red because white consists of various colors.

If you have a red LED, use it in the morning to transition from night to day and in the evening to transition from day to night.

In other words, you can turn the red light on between:

6 pm – 8 pm

6 am – 8 am

You can slot the color into the first two to three hours. Instead of slowly ramping up the brightness of a daylight bulb in the morning until you reach full intensity, use a red LED. Do the same at the end of the day.

Some people only use red lighting at night because it allows them to see the tank’s inhabitants without disturbing the fish.

While some fish can sleep with the red light on, you should give them a few hours of total darkness per night. That is also true for corals, which have their own circadian rhythm.

The same goes for a blue light which shouldn’t be left on all night. This prevents fish from sleeping properly.

Excellent YouTube video explaining the ideal reef tank lighting schedule

Do Corals Need Green And Red Lights?

Corals don’t need green or red lights. They grow best under blue light. Red is a suitable substitute if you don’t have anything at your disposal.

Green is not as useless as aquarists previously thought because it has strong penetrating power.

But the zooxanthellae in corals can do without it. They don’t need green lights to survive in a reef tank.

Does Red Light Make Corals Glow?

Researchers have found that deep-dwelling corals glow in various shades of yellow, orange, and red.^[10]

But exposing corals to a conventional red light won’t make them glow substantially. You’re better off using blue and ultraviolet.^[11]

**A well-maintained reef tank illuminated with red light.**

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.

Conclusions

For those of you in a rush, here is a brief summary of what you should know about red light and aquarium corals:

Zooxanthellae, which live symbiotically with corals, primarily use blue and white light for photosynthesis and there is little evidence to support the use of red light.
Red light doesn’t reach corals in nature because it is absorbed by water bodies. Blue light is present at lower depths of water and is better at penetrating coral tissue.
Some studies have found that red light can harm corals by repressing chlorophyll synthesis and causing necrosis. However, it is still not conclusive and further research is needed.

Conventional reef tanks require two to three hours of low light intensity, followed by eight hours of full intensity, and then two to three hours of low light intensity before the cycle repeats. The use of red light is not specified in this process.