Guides / Lighting

PAR for Reef Tank: How Much Light Do Your Corals Need?

Quick Summary

PAR (Photosynthetically Active Radiation) measures how much usable light reaches your corals. It is the single most objective way to determine whether your lighting is appropriate for the corals you keep. SPS corals generally need 250 to 450 PAR. LPS corals thrive at 75 to 200 PAR. Soft corals and mushrooms do well at 30 to 150 PAR. Every reef tank has a natural PAR gradient from high (near the surface, under the fixture) to low (at the substrate, in the corners), and using this gradient to position corals at their ideal intensity is one of the most effective things you can do for coral health and coloration.

What PAR Actually Measures

When reef keepers talk about light intensity, PAR is the measurement that matters. It quantifies the number of photons in the 400 to 700 nanometer wavelength range that strike a given surface area per second. The unit is micromoles per square meter per second (µmol/m²/s), but most reef keepers simply say "PAR" followed by a number.

A PAR reading of 300 means 300 micromoles of photosynthetically active photons are hitting that spot every second. This is the light energy available for zooxanthellae to drive photosynthesis.

PAR is different from lumens (which measure brightness as perceived by human eyes) and watts (which measure electrical energy consumed by the fixture). A visually dim blue light can deliver higher PAR than a bright white light because blue wavelengths are within the most photosynthetically active range, while green and yellow wavelengths (which our eyes are most sensitive to) contribute less to photosynthesis per photon.

This is why you cannot judge reef lighting by how bright it looks. A fixture that appears blindingly white might deliver less useful energy to corals than a fixture that looks relatively dim and blue. PAR measurement removes this perceptual bias entirely.

Why PAR Matters for Corals

Every coral species evolved at a specific depth range on the reef, and that depth determined the light intensity it received. Shallow-water SPS corals adapted to intense sunlight filtered through only a few meters of water. Deep-water LPS and soft corals adapted to dim, blue-shifted light filtered through 10 to 30 meters. These adaptations are built into their biology.

When a coral receives PAR within its adapted range, zooxanthellae photosynthesize efficiently, the coral produces protective fluorescent pigments at the right density, and calcification proceeds at optimal rates. When PAR is too low, zooxanthellae multiply to capture more light, the coral turns brown, and growth slows because photosynthetic output cannot meet energy demands. When PAR is too high, zooxanthellae produce excess reactive oxygen species that damage coral tissue, triggering the coral to expel them in a bleaching response.

In most tanks, you will notice this pattern clearly. Corals placed too high (too much PAR) pale from the tips. Corals placed too low (too little PAR) darken and stretch their tissue toward the light. Corals at the right PAR show their best natural coloration and grow steadily.

PAR Targets by Coral Type

These ranges represent the practical PAR levels where each coral group performs best in aquarium conditions. Natural reef PAR is often higher than aquarium recommendations because corals on a natural reef receive light for 12+ hours with a gradual intensity curve, while aquarium lights often deliver peak PAR for the entire photoperiod.

Coral Type PAR Range (µmol) Placement Zone
Acropora 300 to 450 Top of rockwork, directly under fixture
Montipora 200 to 400 Upper third, can tolerate less than Acropora
Stylophora, Pocillopora 200 to 350 Upper third
Euphyllia (torch, hammer, frogspawn) 75 to 175 Middle third
Acanthastrea, Micromussa 50 to 150 Mid to lower third
Blastomussa 50 to 100 Lower third, shaded areas
Goniopora, Alveopora 75 to 150 Middle third, moderate flow
Leather corals (Sarcophyton, Sinularia) 75 to 200 Mid tank, adaptable
Zoanthids, Palythoa 75 to 250 Flexible, mid to upper
Mushrooms (Discosoma, Rhodactis) 30 to 100 Lower third, shaded overhangs
Ricordea 50 to 150 Lower to mid third
Tridacna clams 200 to 400 Upper half, direct light

These are working ranges, not rigid limits. Individual specimens vary based on the conditions they were grown under. A Montipora frag grown at 150 PAR at a farm needs acclimation before being placed at 350 PAR in your tank, even though 350 PAR is within the recommended range.

For species-specific detail, see the coral PAR levels guide.

How PAR Varies Inside Your Tank

A single reef tank contains a wide range of PAR values. This gradient is not a problem to solve. It is a feature that allows you to keep corals with different light requirements in the same system.

Top-to-Bottom Gradient

PAR decreases with depth because water absorbs and scatters light. In a typical 24-inch deep tank with a quality LED fixture:

  • Surface (under fixture center): 400 to 600+ µmol
  • Mid-depth (12 inches): 150 to 300 µmol
  • Substrate (24 inches): 50 to 150 µmol

The exact values depend on fixture output, mounting height, and water clarity. But the pattern is consistent: PAR drops roughly 40 to 60% from surface to substrate in most reef tanks.

Center-to-Edge Gradient

PAR is highest directly below the fixture and decreases toward the edges. This is especially pronounced with point-source LED fixtures (puck-style lights) and less dramatic with T5 fixtures or multi-LED bar setups.

In a 48-inch tank lit by a single centered LED:

  • Center (under fixture): 300 µmol at mid-depth
  • 12 inches from center: 200 µmol at the same depth
  • Tank edge (24 inches from center): 80 to 120 µmol at the same depth

Multiple fixtures with overlapping coverage significantly reduce this center-to-edge drop. This is one of the strongest arguments for using two or more fixtures on tanks wider than 30 inches.

Shadows

Rockwork, overhangs, and coral colonies themselves create shadows. An archway in your rock structure might have 250 PAR on top and 30 PAR underneath. These shaded zones are perfect for mushrooms, non-photosynthetic corals, and LPS species that prefer lower light.

How to Measure PAR

The only way to know actual PAR values in your tank is to measure them with a PAR meter (also called a quantum meter). Visual brightness, fixture percentage, and manufacturer specifications all provide estimates, but none account for the specific conditions in your tank.

PAR Meters for Reef Use

Apogee MQ-510: The gold standard for underwater PAR measurement in reef tanks. It uses a waterproof sensor on a cable connected to a handheld meter. Accuracy is excellent, and it is specifically calibrated for aquatic use. The cost is approximately $400 to $500.

Apogee MQ-500: The terrestrial version. It works underwater with a waterproof sensor but is calibrated for sunlight rather than LED spectra. It still provides useful readings but may under-read or over-read depending on the LED spectrum.

Seneye Reef: A more affordable monitoring device that measures PAR, spectrum, temperature, and other parameters continuously. It provides useful relative measurements and trend data. Less accurate than the Apogee for absolute PAR values, but adequate for most reef keepers.

Rental or club sharing: Many local reef clubs own PAR meters that members can borrow. This is the most cost-effective option if you only need occasional measurements. Some fish stores also offer PAR meter lending or measurement services.

How to Take Measurements

When mapping your tank with a PAR meter, follow a consistent process:

  1. Run your lights at normal daytime intensity settings.
  2. Position the sensor face-up (measuring light coming from above) at each measurement point.
  3. Measure at a grid of locations: center surface, center mid-depth, center substrate, and repeat at the edges and corners.
  4. Record each reading along with the location and depth.
  5. If possible, measure at the specific locations where you plan to place corals.

Hold the sensor steady for 3 to 5 seconds at each point to allow the reading to stabilize. Avoid casting your shadow over the sensor during measurement.

Creating a PAR Map

A PAR map is a visual representation of light intensity across your tank. You can draw it on paper, use a spreadsheet, or simply note values on a photo of your tank.

A basic PAR map might look like:

Depth Left Edge Center Right Edge
Surface 250 450 220
Mid (12") 130 280 110
Substrate (24") 60 140 50

This map tells you exactly where to place each coral type. Acropora goes at the center top (450 PAR). Euphyllia goes mid-depth at the edges (110 to 130 PAR). Mushrooms go at the substrate edges (50 to 60 PAR).

Using PAR to Place Corals

Once you have a PAR map, coral placement becomes systematic rather than guesswork.

High PAR Zone (250+ µmol)

This is SPS territory. Place Acropora, Montipora, Stylophora, and Tridacna clams here. These organisms need intense light for efficient photosynthesis and fast calcification. If you notice SPS corals browning in this zone, the issue is likely nutrients (too high nitrate or phosphate), not lighting.

Medium PAR Zone (100 to 250 µmol)

LPS corals, zoanthids, and some soft corals thrive here. Euphyllia, leather corals, and Goniopora are well suited to this range. This zone often represents the mid-height of the rockwork or the areas near the edges under the fixture.

Low PAR Zone (30 to 100 µmol)

Mushrooms, Blastomussa, non-photosynthetic corals, and low-light LPS go here. This zone includes the substrate, shaded overhangs, and the far edges of the tank. Many beautiful corals actually prefer low light, so do not treat this zone as wasted space.

Acclimation Between Zones

When moving a coral to a higher PAR zone, do it gradually. Place the coral one "step" higher in the rockwork and leave it for one to two weeks. If it shows good polyp extension and no signs of stress, move it another step. Jumping a coral from 80 PAR to 350 PAR in a single move is one of the most common causes of bleaching in home reef tanks.

For detailed placement strategies, see the coral placement guide.

Factors That Affect PAR in Your Tank

Several factors beyond fixture output influence the PAR that actually reaches your corals.

Fixture Height

PAR decreases with distance from the source following the inverse square law. Raising a fixture from 8 to 12 inches above the water reduces PAR by approximately 44%. Lowering it from 12 to 8 inches increases PAR by approximately 78%. Small height changes have outsized effects.

Water Clarity

Dissolved organics, particulates, and algae in the water column absorb and scatter light. Clean, crystal-clear water transmits more PAR to depth. Running activated carbon, maintaining a good protein skimmer, and performing regular water changes improves light transmission.

In practice, a tank with yellowed water from dissolved organics can lose 10 to 20% of PAR at depth compared to crystal-clear water. This loss is invisible to the eye but measurable with a PAR meter.

Salt Creep and Fixture Cleanliness

Salt spray accumulates on the lens of LED fixtures over time, reducing light output. Wipe fixture lenses weekly with a damp cloth. A dirty lens can reduce PAR output by 5 to 15% without any visible change in the light's appearance.

LED Degradation

LED chips lose output over time. Most quality LEDs maintain 80 to 90% of original output after 50,000 hours (approximately 6 years at 10 hours per day). If corals are gradually declining after several years with the same fixture, measure PAR to check whether LED degradation has reduced intensity below useful levels.

Water Surface Agitation

A calm water surface acts like a lens, focusing and defocusing light as it ripples. Heavy surface agitation can slightly reduce average PAR by scattering light sideways. However, the natural shimmer effect from surface ripples also creates brief moments of concentrated PAR (called light flecking) that may actually benefit corals by simulating natural reef conditions. In most tanks, surface movement is a net neutral or slight positive for PAR delivery.

System Interactions

Spectrum

PAR measures total photon count across the 400 to 700 nm range, but not all wavelengths are equally useful for coral photosynthesis. Blue wavelengths (440 to 480 nm) are the most photosynthetically efficient for zooxanthellae. A fixture delivering 300 PAR with a blue-heavy spectrum provides more usable energy to corals than a fixture delivering 300 PAR with a white-heavy spectrum. See the reef light spectrum guide for wavelength detail.

Nutrients and Coloration

PAR interacts with nutrient levels to produce coral coloration. At appropriate PAR with low nutrients (nitrate 2 to 5 ppm, phosphate 0.02 to 0.05 ppm), corals display their most vivid fluorescent pigmentation. High PAR with high nutrients causes zooxanthellae overgrowth and browning. The best coral color comes from matching PAR to the coral's needs while keeping nutrients controlled. See the nitrate guide.

Calcification

Light directly drives calcification rate. Corals calcify faster during the day under strong light because photosynthesis provides the energy needed for active calcification. Adequate PAR is a prerequisite for healthy calcium and alkalinity consumption. If your tank consumes less calcium and alkalinity than expected for its coral load, measure PAR to determine whether lighting is a limiting factor.

Photoperiod

PAR and photoperiod together determine the total daily light energy (DLI, Daily Light Integral) that corals receive. High PAR for 8 hours delivers a different total energy than moderate PAR for 12 hours. For reef tanks, higher PAR for a shorter period (8 to 10 hours) is generally preferable to lower PAR for a longer period, because extended photoperiods benefit algae more than corals. See the photoperiod guide.

Advanced: Daily Light Integral (DLI)

While PAR is an instantaneous measurement (photons per second), DLI measures the total photon delivery over an entire day. It is expressed as moles per square meter per day (mol/m²/d).

DLI = PAR × photoperiod (in seconds) ÷ 1,000,000

For example, a coral receiving 200 PAR for 10 hours: DLI = 200 × 36,000 ÷ 1,000,000 = 7.2 mol/m²/d

Natural reef environments typically deliver 5 to 15 mol/m²/d depending on depth, season, and water clarity. Most reef tanks fall in the 5 to 12 mol/m²/d range, which aligns well with the needs of common aquarium corals.

DLI is useful for comparing different lighting strategies. A coral receiving 150 PAR for 12 hours gets 6.5 mol/m²/d. The same coral receiving 250 PAR for 8 hours gets 7.2 mol/m²/d. The DLI is similar, but the shorter, more intense schedule is typically better for reef tanks because it reduces the competitive advantage of algae during extended low-light hours.

Advanced: PAR Meter Accuracy and LED Spectra

Not all PAR meters read LED light accurately. Most PAR meters are calibrated for natural sunlight, which has a broad, relatively even spectrum. LED fixtures produce light in narrow spectral peaks (particularly in blue and white channels), which can cause certain PAR meters to over-read or under-read by 10 to 30%.

The Apogee MQ-510 includes correction factors for common LED spectra and is the most reliable meter for reef LED measurement. The Seneye Reef uses spectral analysis rather than a broadband sensor, which can provide more accurate readings under narrow-band LED light.

If you are using a standard (non-reef-calibrated) PAR meter, be aware that readings may not be perfectly accurate under heavily blue LED light. The relative readings (comparing one location to another within your tank) remain useful even if the absolute values are slightly off.

For practical purposes, the directional information from any quality PAR meter (where is PAR highest, where is it lowest, and how does it change with depth) is more valuable than perfect absolute accuracy. Use the readings to place corals relatively, and adjust based on coral response over time.

Common Myths

"Higher PAR always means better coral growth." Every coral has an optimal PAR range. Exceeding it does not accelerate growth and can cause bleaching. An Acropora at 500 PAR does not grow faster than one at 350 PAR, but it is at greater risk of photodamage.

"You can judge PAR by how bright the light looks." Human eyes are most sensitive to green and yellow wavelengths, which are not the most useful for coral photosynthesis. A visually bright white light often delivers less photosynthetically useful PAR than a dimmer blue-violet light.

"PAR at the surface is what matters." PAR at the coral's location is what matters. Surface PAR can be 500+ while substrate PAR is 60. Always measure or estimate PAR at the specific depth where your corals sit.

"All corals need high PAR." Many beautiful corals thrive at low PAR. Mushrooms, Blastomussa, and deep-water LPS species can bleach and die under intense light that SPS corals would thrive under. Matching PAR to coral type is more important than maximizing PAR everywhere.

"A PAR meter is too expensive to be practical." Many reef clubs loan meters to members, and some fish stores offer measurement services. Even a single measurement session provides information that guides coral placement for months. The cost per insight is low when shared or rented.

FAQ

What PAR do I need for a mixed reef?

A range of 75 to 300 PAR across different zones in the tank covers most mixed reef coral types. Place SPS at 250 to 300 PAR (top), LPS at 75 to 175 PAR (middle), and soft corals at 50 to 100 PAR (bottom).

How do I measure PAR without a PAR meter?

You cannot measure PAR without a PAR meter. However, you can estimate relative PAR using manufacturer data, fixture height charts, and online PAR databases where other users have shared readings from specific fixtures at specific settings. These estimates are approximate but better than nothing.

Does PAR change over time?

Yes. LED chips degrade slowly, losing 10 to 20% output over 50,000 hours. Salt creep on lenses reduces output. Water clarity changes affect PAR at depth. Remeasure PAR annually or whenever you notice changes in coral behavior.

Can I have too much PAR?

Yes. PAR above a coral's optimal range causes photodamage and bleaching. SPS corals can generally handle up to 500 PAR, but above that, most species show stress. LPS and soft corals can bleach at PAR levels that SPS would thrive under.

What PAR do I need for coralline algae?

Coralline algae grows across a wide PAR range (30 to 300+ PAR). It is less demanding than stony corals and will colonize any surface with sufficient light and available calcium and alkalinity.

Is PAR the same under LEDs and T5s?

PAR is PAR regardless of the light source. However, the distribution differs. LEDs create concentrated PAR under the fixture with rapid falloff. T5s distribute PAR more evenly across the tank. The same average PAR number from different technologies produces different spatial patterns.

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