Reef Light Spectrum for Soft, LPS and SPS Corals

Two fixtures can both claim 14,000K on the box and look nothing alike over the same coral. Reef light spectrum is the actual distribution of wavelengths a light emits, and it decides whether a Montipora goes pastel or brown, whether zoanthids glow, and how much usable energy reaches the algae living inside coral tissue. Color temperature only summarizes how white light appears to a human eye. This article stays on spectrum alone, because intensity, PAR and fixture selection are separate questions that deserve their own answers.

Reef light spectrum is not the same thing as a Kelvin rating

Correlated color temperature describes appearance. A 6,500K light looks neutral or slightly warm, 10,000K reads as crisp white, and 20,000K looks distinctly blue. What that single number cannot tell you is where the energy actually sits. Two fixtures rated identically can put out completely different amounts of violet, royal blue, green and red, and corals respond to those individual bands rather than to the average impression they create together in a room.

Modern reef lighting is built from channels, and the channels are the real story. Most LED systems combine royal blue near 450 nanometers, blue closer to 470, violet somewhere between 400 and 420, cool and warm white, and often separate green and red emitters. T5 lighting shapes the same curve by mixing tube types across the fixture. Whichever technology you run, the useful question is which bands dominate the output, not what the marketing name on the housing says.

Manufacturers who publish an honest spectral output chart make this easy, and that chart is worth more than any product description. Look for where the peaks fall and how tall they are relative to each other. The word actinic gets used loosely for anything roughly in the 420 to 460 range, so it tells you very little on its own. Corals never see a label; they see photons at specific wavelengths, absorbed by specific pigments.

How corals actually use blue, violet and red light

The symbiotic algae inside coral tissue run photosynthesis with chlorophyll a and c2 alongside accessory pigments such as peridinin. Those pigments absorb strongly through the blue and blue-green range, with a smaller secondary response in deep red. On a natural reef, red and orange wavelengths are absorbed within the first few meters of water, so corals living at ten or twenty meters evolved under light that is overwhelmingly blue. That history is why blue-dominant lighting suits them so well in a glass box.

Color is a different mechanism from growth, and this is where people get confused. The greens, oranges and reds that make a coral glow come from fluorescent proteins that absorb violet and blue light and re-emit it at longer wavelengths. Purples and deep blues usually come from chromoproteins, which are not fluorescent at all and often become more strongly expressed under high-energy light. Violet and royal blue are therefore doing double duty, feeding photosynthesis and driving the visual payoff at once.

Green and red emitters mostly serve your eyes rather than the corals. They render fish color, coralline algae and the reds in an Acanthastrea honestly instead of leaving them muddy. Many reef keepers also find that running white, green and red channels hard encourages nuisance algae, so most tanks end up using those bands as seasoning rather than as the main course.

Blue to white balance decides both growth and how the tank reads

Push the balance heavily toward blue and violet and the tank turns into a fluorescence display. Pastels intensify, chromoprotein purples deepen, and the whole system looks like a night dive. The cost is realism: whites wash out, fish lose their natural color, and detail flattens because your eye receives a very narrow slice of the spectrum. Corals do not object, though anyone photographing the tank usually finds those images impossible to correct later.

Add more white and the picture reverses. Broader output renders color naturally and gives the algae inside coral tissue more of what they can use, which frequently means faster growth. It also usually means browner corals, because zooxanthellae density rises when there is plenty of usable light, and denser algae mask the coral's own pigments. That is not a health problem, though it is the most common reason somebody says their colonies lost color after a lighting change.

Most successful mixed reefs land on a blue-dominant baseline with enough white to keep the display honest, then lift the whites for a few hours in the evening when people are actually watching. Photoperiod and ramp shape sit alongside spectrum in that decision, and total light delivered matters at least as much as its color. Our broader guide to reef lighting choices covers intensity and fixture questions that this article deliberately leaves alone.

Reef light spectrum by coral type: soft, LPS and SPS

Soft corals are the most forgiving group and will grow across a wide range of spectra. Sarcophyton and Sinularia leathers, Discosoma and Rhodactis mushrooms and green star polyps all do well under a blue-dominant mix without fuss. Zoanthids and palythoas are the exception worth tuning for, because their oranges, pinks and greens are fluorescent pigments that violet excites directly. A colony that looks dull under a white-heavy fixture often transforms once violet output comes up.

These are the starting balances we work from before adjusting for a specific tank:

• Soft corals and mushrooms: blue-dominant with modest white, and they tolerate the widest spectral range in the tank

• Zoanthids and palythoas: add violet, since their brightest pigments are fluorescent and violet excites them

• Euphyllia, Duncan and other flowing LPS: blue-heavy for fluorescence, with enough white that the tissue does not read flat

• Acanthastrea, Micromussa, Favia and Blastomussa: a measured amount of green and red renders their reds truthfully

• Acropora, Montipora, Stylophora and Seriatopora: blue-dominant with violet plus steady white, which is where pastels come from

Large polyp stony corals sit in the middle of the difficulty range and reward blue-forward spectra. Hammer, torch and frogspawn corals fluoresce beautifully under blue and violet, and their tips often develop stronger contrast when violet is present. The chalice and acan crowd behaves differently: those reds and oranges are far more satisfying with a little red rendering in the mix, and a tank running pure blue tends to hide exactly the colors people paid for.

Small polyp stony corals are where spectrum is least forgiving. Acropora and Montipora shift color visibly with the balance you choose, and the same colony can look pastel blue under one setting and dull tan under another with no change in health. Blue-dominant output with real violet content, backed by consistent white, is what most people converge on. Our comparison of what LPS and SPS corals each demand covers the husbandry side of that difference.

Changing reef light spectrum safely takes weeks, not an evening

Corals photoacclimate slowly. They adjust zooxanthellae density and pigment expression over a period of weeks, which means a spectrum change made on a Saturday can still be playing out a month later. Move too fast and you get paling, tissue recession or a sudden brown-out, and none of those reverse quickly. Change one variable at a time, write down what you changed, and give the tank at least two weeks to answer before you touch anything again.

A slow ramp is genuinely the whole technique. Adjust channel output by small increments weekly, watch polyp extension, tissue color and the tips of any SPS colonies, and hold whatever setting produces steady behavior. Keep alkalinity stable while you do it, because corals handle a lighting change far worse when chemistry is drifting at the same time. Sudden alkalinity swings paired with a spectrum shift are a reliable way to burn tips on Acropora.

Aging lamps quietly move your spectrum without asking. Metal halide bulbs shift in color and lose output over a year, and T5 tubes drift in the same direction, which is why both get replaced on a schedule rather than when they fail. LED emitters degrade more slowly but not infinitely, and different channels age at different rates. Keeping a record of settings and replacement dates turns an invisible drift into something you can actually manage.

Key takeaways

• Reef light spectrum describes which wavelengths a fixture emits; Kelvin only describes how the mixture looks.

• Blue and violet drive both photosynthesis and the fluorescent pigments that make corals glow.

• More white generally means faster growth and browner tissue, because zooxanthellae density rises with usable light.

• Soft corals tolerate almost anything, LPS reward blue-forward light with some red rendering, and SPS color shifts most sharply.

• Corals photoacclimate over weeks, so spectrum changes belong in small weekly steps with everything else held stable.

Frequently asked questions

Is a higher Kelvin rating better for coral color?

Not inherently. A higher rating means the light looks bluer, which usually does improve fluorescence, but two fixtures at the same rating can deliver very different violet and royal blue content. Judge a light by its spectral chart and by how your corals respond over several weeks, not by the number on the box.

Can corals grow under blue light alone?

They can, and many tanks run close to it, but the results are mixed. Blue drives photosynthesis efficiently, though most keepers find that a small amount of white produces sturdier growth and a display that still looks like a reef rather than a blacklight poster. Pure blue also makes health problems harder to spot early.

Will adding red and green light cause an algae problem?

It can contribute, particularly on a tank already carrying elevated nutrients. Broader spectrum benefits nuisance algae as well as corals, so those channels are best used sparingly and mainly during viewing hours. If algae appears after a spectrum change, treat nutrients and export as the first suspects and the lighting as the second.

Work with Marine Concepts Inc

If you are unsure what your fixture is actually putting out, bring your channel settings and a recent photo of the tank and we will work through the balance with you. You can read about our local aquarium shop before visiting Marine Concept Supply in Wheeling, IL, where coral, lighting and supplies are available for same-day pickup, and calling ahead is the surest way to confirm what is on hand.