Reef Aquarium Lighting and PAR

By the Coral Compass team · Last reviewed 2026-10-15

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Why light matters

Most reef corals host single-celled algae called zooxanthellae (family Symbiodiniaceae) inside their tissue. The algae photosynthesize and pass much of the sugar they make to the coral, which for many species supplies most of its daily energy. The coral provides shelter, carbon dioxide and nutrients in return. Too little light and the coral starves or browns out as it packs in more algae. Too much, especially suddenly, and the photosynthetic machinery is damaged and the coral expels its algae: bleaching.

PAR, PUR and why lux is useless

PAR (photosynthetically active radiation) is the amount of light in the 400–700 nm wavelength band that photosynthesis can use. It's measured as photosynthetic photon flux density in µmol photons per square meter per second (µmol/m²/s). That's the number reef lighting charts talk about.

Lux and lumens, which you'll see on household bulbs, are weighted to human vision, which peaks in green-yellow light. Reef LEDs are heavy in blue and violet, which look dim to us but are very usable for corals, so lux badly underestimates reef lights.

PUR (photosynthetically usable radiation) is a marketing-era term for the part of PAR that corals' pigments absorb best, mostly blue (roughly 400–500 nm) with some red. It isn't measured by ordinary meters. In practice, reef-spectrum fixtures are blue-heavy, so PAR is a good working measure.

PAR targets by coral type

These ranges come from a broad consensus of hobby measurements and manufacturer guidance, and individual colonies acclimate to a range. Corals collected from shallow reef flats can see well over 1,500 µmol/m²/s at noon in the wild, but aquarium corals do well at a fraction of that because they get it for many hours with no clouds.

Daily light integral (DLI) is another way to think about it: total photons delivered over the day. A tank at 250 PAR for 8 hours gives the same daily dose as one at 200 PAR for 10 hours. That's why some tanks with longer photoperiods run lower peak intensity.

Measuring PAR

A quantum PAR meter with a submersible sensor gives real numbers. The Apogee MQ-510 is widely used because it's designed to read LED spectra accurately underwater. Many local reef clubs and stores lend meters. To map a tank:

1. Run lights at your peak program.
2. Measure at several heights (top rocks, mid-level, sand) and at the center and corners.
3. Sketch a map and place corals by their needs.

PAR falls off quickly with depth and distance, and LEDs can create hot spots directly under the fixture. Manufacturers publish PAR maps, but those are usually measured in ideal conditions. Measure your own if you can.

Spectrum and Kelvin

Color temperature in Kelvin (K) describes the overall look of light. Most reef tanks look their best somewhere between 10,000 K and 20,000 K. Higher Kelvin looks bluer. Modern LEDs mix channels (royal blue, blue, violet/UV, white, sometimes red and green), so "Kelvin" just describes the blend.

Blue and violet light (around 400–470 nm) drive photosynthesis in zooxanthellae efficiently and make fluorescent pigments glow. A spectrum that is mostly blue with some white is a sensible default. Very heavy white or red light tends to favor algae and doesn't look natural on a reef.

Photoperiod

A typical schedule is 8–10 hours of usable light per day, plus optional dim blue ramps at the start and end for viewing. Many reefers run around 6–8 hours at or near peak intensity. Avoid very long peaks: they add little benefit and more algae. Consistency matters, so use a timer or the fixture's controller.

Fixture types

LED

LEDs dominate today. They're efficient, programmable, run cool, and let you tune spectrum and intensity per channel. Downsides are potential hot spots and shadowing from point sources, and the temptation to run them too strong. Spread multiple fixtures or use wide-angle optics for even coverage.

T5 fluorescent

T5 HO tubes spread light very evenly and have a strong record with SPS. Tubes lose intensity over time and are typically replaced every 9–12 months, which adds running cost.

Hybrid

Hybrids combine T5s and LEDs, getting T5 evenness plus LED punch and control. They're popular on SPS tanks but cost more and run warmer.

Metal halide

Once the standard for SPS, metal halide is now uncommon because of heat and power use, though it produces excellent results.

Acclimating corals and new lights

Most light-related coral losses come from sudden change. Whenever you change fixtures, raise intensity, or add a coral from a dimmer system:

• New fixture: start at roughly 50% of your target intensity (some start lower) and increase by a few percent every few days over 4–6 weeks.
• New coral: place it lower in the tank or shade it, then move it up over a couple of weeks. Store corals often sit under modest light.
• After maintenance or a light outage: resume normal programs; brief darkness does not hurt corals.

Reading the signs

• Too much light: bleaching (pale or white tissue) on upper surfaces, receding tissue at the top, closed polyps, LPS shrinking.
• Too little light: browning out, slow growth, LPS and softies stretching toward the light.

Lighting is never the only variable. Browning can also come from high nutrients, and bleaching can follow temperature spikes or alkalinity swings. Rule out water chemistry with the dosing calculator and your test logs before blaming the lights.

Common lighting mistakes

• Running a new LED at 100% on day one. This is the classic cause of mass bleaching after an upgrade. Ramp up over weeks.
• Trusting a fixture's marketing PAR. Published numbers are often measured directly under the fixture at short distances. Your corners and sand may get a fraction of that.
• Changing several things at once. If you change spectrum, intensity and photoperiod together and corals react, you won't know which caused it. Change one variable at a time.
• Chasing color with more light. Coral color depends on light, nutrients, flow and stability together. More intensity often produces paler, not brighter, corals.
• Ignoring the glass and lenses. Salt creep on lenses and algae film on the water surface or cover glass can cut PAR noticeably. Clean them regularly.
• Forgetting heat. Strong fixtures, especially metal halide and hybrids, can raise tank temperature. Monitor temperature after any lighting change, because heat stress bleaches corals just as light stress does.

Lighting and flow also work together. High-light corals generally need strong flow to carry away oxygen and waste produced by intense photosynthesis and to deliver food. A coral in bright light but stagnant water often does worse than one in moderate light with good flow.

Summary

Use PAR, not lux. Aim roughly for 50–150 PAR for softies, 75–200 for LPS, and 250–500 for demanding SPS, measured with a quantum meter. Run 8–10 hours with a blue-heavy spectrum, and acclimate new lights and new corals over weeks rather than days.

Frequently asked questions

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More reef science guides

• Alkalinity, calcium & magnesium
• Nitrogen cycle & cycling
• Nitrate & phosphate
• Salinity & RO/DI
• Coral types & care
• Pests & diseases
• Reef conservation & science
• Coral symbiosis & Symbiodiniaceae
• Coral bleaching science
• Ultra-low nutrient research
• Ocean acidification & calcification
• Coral restoration & microfragmentation
• ICP testing & trace elements

Ranges are common hobby targets compiled from reef chemistry literature (including Randy Holmes-Farley's articles), hobby consensus and manufacturer documentation. They're not rules. Last reviewed 2026-10-15.