PAR Explained: How Much Light Does Your Coral Actually Need?

PAR (Photosynthetically Active Radiation) is the measurement that actually tells you how much usable light your coral receives at its specific location in the tank. Watts and Kelvin, despite appearing on every fixture spec sheet, cannot answer that question.

Why Watts and Kelvin Do Not Tell You What Your Coral Is Getting

Wattage is a measure of power draw from the wall. A fixture consuming more watts is not automatically delivering more usable light to your coral; it is simply using more electricity. The relationship between power consumption and light output varies widely between fixture designs, diode quality, and reflector geometry.

Kelvin describes color temperature, the visual warmth or coolness of the light. A 20,000K fixture looks blue and crisp; a 10,000K fixture looks whiter. Neither rating tells you anything about intensity. Two fixtures with identical Kelvin ratings can produce dramatically different amounts of usable light at coral depth.

Relying on either spec to set up a reef is like judging how loud a speaker is by looking at its color. The number is real, it just does not measure the thing you need to know.

What PAR Actually Measures

PAR measures the actual quantity of light photons in the wavelength range that drives photosynthesis, arriving at a specific point in your aquarium. The key phrase is "specific point." PAR is not a property of the fixture; it is a property of a location in the water column, measured with a PAR meter held at coral placement depth.

This distinction matters enormously in practice. The same fixture that delivers one PAR reading directly below it at shallow depth will deliver a very different reading at greater depth. Water absorbs and scatters light, so the drop-off with depth is real and significant. A reading taken at the surface, or estimated from a manufacturer's spec sheet measured in air, tells you very little about what your Acropora frag is actually receiving on the sand bed.

The only reliable way to know your PAR at a given spot is to measure it there, with a meter, under your actual water conditions.

PAR Target Ranges for Different Coral Types

These ranges reflect what different coral categories generally use well. They are starting points and context, not precise targets to chase obsessively.

• Soft corals and LPS: 50 to 150 PAR
• Mixed reef and moderate SPS: 150 to 250 PAR
• High-light SPS including Acropora: 250 to 450 PAR, depending on species and how well acclimated the coral is

Soft corals and most LPS are tolerant and adaptable. They can often thrive across a wider range than the numbers suggest, provided the transition to any new light level is gradual. Mixed reef systems running a combination of LPS and less demanding SPS sit comfortably in the middle range. High-light SPS, particularly branching Acropora, are the corals most associated with the upper end of these ranges, but even within that genus, acclimation state matters as much as the absolute number.

The Bigger Risk Is Moving Coral Too Fast, Not Too Little Light

Modern LED fixtures are powerful. Many hobbyists running a mid-range or high-end LED unit are producing more than enough raw light output to meet even demanding SPS requirements. The more common problem is not under-lighting; it is photo-stress from moving a coral into high PAR too quickly.

Photo-stress presents as bleaching. The coral expels its zooxanthellae and loses color, beginning at the tips of branches or the top of a colony where light intensity is greatest. Once a coral starts bleaching from light shock, recovery requires immediately reducing intensity or moving the coral to a shadier position, then rebuilding slowly.

This outcome is preventable. The coral did not fail; the acclimation process did.

How to Acclimate New Frags to Your Light

The practical method is straightforward: start low, ramp gradually, and watch the coral.

1. Place new frags in a lower-light area of the tank. The sand bed, a shaded corner, or a position farther from the fixture all work. The goal is to match or stay below the PAR the frag was raised under at the source tank or retailer, which you likely do not know exactly, so erring low is safer.
2. Hold that position for several days to a week while the coral settles, opens its polyps consistently, and shows stable color. A closed or retracted coral is telling you something is off, whether that is light, flow, water chemistry, or stress from the move itself.
3. Move the frag gradually toward its target position over two to three weeks. Small incremental moves every few days allow the coral's zooxanthellae population to adjust to increasing light intensity without triggering bleaching.
4. Watch color and polyp extension throughout. Browning out can indicate the coral is getting too little light and ramping up zooxanthellae density. Bleaching or pale tips indicate too much, too fast. Both are feedback signals worth more than any forum post about what PAR number worked in someone else's tank.

Using a PAR Meter Effectively

If you have access to a PAR meter (many local fish stores lend them, and they appear in reef club loaner programs), take readings at multiple depths and positions throughout your tank, not just under the center of the fixture. Edges, corners, and spots behind rockwork will read considerably lower than the most exposed central position at the same depth.

Map out your tank once, then use that map to intentionally place corals. High-light SPS go toward the top and center. LPS and softies fill the edges, lower shelves, and shaded nooks. This approach turns your lighting gradient from a limitation into a planning asset.

If you do not have a PAR meter, coral behavior is still a usable guide. Consistent polyp extension during photoperiod and stable, species-appropriate color are signs the coral is processing light well. Chronic recession, persistent retraction, or slow tissue loss alongside otherwise stable water parameters are worth investigating with a meter before assuming a chemistry cause.

What Affects PAR Beyond the Fixture Setting

Several factors reduce PAR between the fixture and the coral, and they are worth keeping in mind when a meter reading surprises you.

• Water clarity: Dissolved organics, tannins, and suspended particles all absorb light. A tank with yellow-tinted water runs meaningfully lower PAR at depth than a crystal-clear system running the same fixture at the same setting.
• Glass and acrylic thickness: Covers and lids attenuate light. A full glass lid can reduce PAR noticeably compared to an open top.
• Fixture age: LED diodes lose output gradually over time. A fixture running at the same percentage setting as two years ago may be delivering less PAR than it did when new.
• Lens and reflector fouling: Salt creep and dust on fixture lenses or reflectors reduce output. Cleaning the optics periodically keeps output consistent.

Frequently Asked Questions

Can I use wattage per gallon as a guide for reef lighting?

Wattage per gallon was a useful rough guide in the fluorescent era, but it does not translate reliably to modern LED systems. LED fixtures vary too widely in efficiency and optical design for wattage to predict light output at coral depth. PAR measurement at the coral's actual location is the only reliable method.

How often should I remeasure PAR in my tank?

A baseline measurement when you set up or change a fixture is essential. After that, remeasuring makes sense if you change water clarity significantly, add a new coral or rearrange rockwork, notice unexplained coral behavior changes, or suspect fixture output has declined with age. There is no fixed schedule; measure when something has changed.

Is higher PAR always better for SPS?

No. PAR above a coral's acclimated tolerance causes photo-stress and bleaching even in high-light species like Acropora. The target range for high-light SPS tops out at 450 PAR in the original guidelines, and even within that range, the coral's acclimation history determines what it can handle. More is not automatically better.

Why does my coral look brown even under strong light?

Browning often indicates the coral is shading itself by increasing zooxanthellae density, a response to insufficient light. If your PAR reading at that position falls below the appropriate range for the species, moving it to a brighter location gradually should improve coloration. Other causes include elevated nutrients, but light is the first variable to check.

What is the difference between PAR and PUR?

PAR measures the total quantity of photons in the range used for photosynthesis. PUR (Photosynthetically Usable Radiation) narrows that to the specific wavelengths zooxanthellae actually absorb most efficiently. The original post focuses on PAR as the practical measurable standard; PUR is a refinement that matters more in fixture selection than in day-to-day coral placement decisions.

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If you want to log your PAR readings alongside water parameters and coral health observations in one place, TankMind is built exactly for that kind of ongoing reef record-keeping.