Light. Why your eye is a liar

I want to carry on with this series to reflect a little and bring these articles to everyone, because sometimes I think we rely too much on "they say so"…

I am not a professor, a know-it-all or a "keyboard warrior": my aim is to understand. And to understand, you have to step a little outside the "crowd's" way of thinking.

Why your eye is a liar

If you are one of those who runs the white LEDs at 100% or wants to see the tank "white"… "because that way I can see better whether my Acropora are clean or coloured", I do not agree with you: you are looking at your tank through the eyes of a land primate 🤣, not those of a marine organism.

Let's sit down, have a coffee and try to understand why what the human eye likes is awful for your corals.

The great deception: evolution versus the reef

The human eye is an extraordinary machine, but it was designed for life on land. We are extremely sensitive to green and yellow (around 550 nm).

When you blast "white" into the tank your retina rejoices, but it is an illusion.

According to Gregory Skomal, PhD (Saltwater Aquariums For Dummies), the light visible to us is only a fraction of what the coral needs. The coral does not "see" brightness: it feels the energy of the photons.

1. 1Violet 400-420 nm switches on fluorescent proteins: colour and defence
2. 2Royal blue 450 nm the fuel for the zooxanthellae's chlorophyll
3. 3Red 650-660 nm in the sea it vanishes first, in the tank it drives algae
4. 4Human eye sees green and yellow best, the light the coral needs least

The eye's curve peaks where the coral draws least.

What the human eye sees and the bands of light the coral uses.

The physics of extinction: red is the first ghost

In the sea, red light (650-660 nm) is the first to die. As documented by Martin Hovland in his key book Deep-Water Coral Reefs: Unique Biodiversity Hot-Spots, water acts as a filter (like when we had ADSL at home and had to fit the filter… the kids born after 2000 will never know 🤣).

At just 5 metres deep the red spectrum is almost completely filtered out. In the aquarium, using white LEDs loaded with red makes no sense: it stimulates algal growth and oxidative stress, but not the "good" photosynthesis.

1. 1Red almost gone by 5 metres
2. 2Yellow fades a little lower down
3. 3Green lasts longer
4. 4Blue the colour that reaches deepest

At 5 metres red has almost gone. The reef lives in blue light.

Sources: [2]

With depth the colours disappear: red first.

PUR versus PAR: let's stop chasing random numbers

PAR measures how much light there is.

PUR (Photosynthetically Usable Radiation) measures how much of that light the coral can use.

Recent research (such as the study published in Nature Scientific Reports, 2025) confirms that the "keys" to life for an SPS are:

• 400-420 nm (violet/UV): it is not there to show off on Instagram. It stimulates the synthesis of fluorescent proteins (FP). Without this burst of energy the coral does not switch on its protective shields (biomineralisation and photoprotection, Farmer et al., 2015).
• 450 nm (royal blue): the peak of chlorophyll absorption. It is pure fuel.

1. 1PAR how much light there is in the tank
2. 2Violet 400-420 nm switches on fluorescent proteins
3. 3Royal blue 450 nm the fuel for chlorophyll
4. 4Green and yellow the coral needs them less
5. 5Red in the tank it drives algae

PUR is only the part of the light the coral can use.

The sarcasm of "white light for health"

"Yes, but under the whites I can see if it is brown!". Dear friend, if your coral is brown the problem is not the light, it is nutrient management. Brown is chlorophyll: under the whites you only see how "loaded" the coral is with symbiotic algae.

Under the blues/UV (wide band blue), on the other hand, you see the coral's immune response. As Prof. Yuri I. Sorokin explains (Coral Reef Ecology), fluorescence is a metabolic investment: if it glows, the coral is producing defences. If it looks "light and clean" under the whites, it is often just an underfed coral in a system that is too lean.

Under the whites

• You see how loaded it is with symbiotic algae
• Brown is chlorophyll, not the light's fault
• Light and clean often means underfed

Under blues and UV

• You see the coral's response
• Fluorescence is an investment
• If it glows, it is producing defences

If the coral is brown the problem is not the light, it is the nutrients.

Remember the legendary tanks with 14,000-20,000 K HQIs or actinic T5s? They were not beautiful because they were "white". They were amazing because those lamps had monstrous peaks of UV and violet. Modern broad-spectrum fixtures (whether Orphek, Radion or CoralCare) have not invented anything: they have finally reproduced with LEDs that spectral "violence" that Acropora need.

In short

Let's stop trusting our retina. Modern fixtures do not emit blue light because it is fashionable, but because we have finally accepted what science has been saying for years: the reef is blue.

Sources cited by the author

• Sorokin, Y. I. (1995). Coral Reef Ecology. Springer.
• Hovland, M. (2008). Deep-Water Coral Reefs. Springer-Praxis.
• Skomal, G. (2020). Saltwater Aquariums For Dummies. Wiley.
• Nature Scientific Reports (2025). Coral response to LED spectra and pigmentation.
• Farmer, J. R., et al. (2015). Effects of seawater-pH and biomineralization on corals. Geochimica et Cosmochimica Acta.

Who wrote this article

Sources

1. Wijgerde T., van Melis A., Silva C.I.F., Leal M.C., Vogels L., Mutter C., Osinga R. (2014). Red Light Represses the Photophysiology of the Scleractinian Coral Stylophora pistillata. PLOS ONE 9(3): e92781.
2. NOAA Ocean Exploration (2020). Why are so many deep-sea animals red in color?. National Oceanic and Atmospheric Administration.
3. Farmer J.R., Hönisch B., Robinson L.F., Hill T.M. (2015). Effects of seawater-pH and biomineralization on the boron isotopic composition of deep-sea bamboo corals. Geochimica et Cosmochimica Acta 155: 86-106.