What Is PAR?
PAR is the usable light corals and plants photosynthesize with, measured 400 to 700nm. Learn how PAR differs from lumens and what targets to aim for.
PAR stands for photosynthetically active radiation, the amount of light in the 400 to 700 nanometer range that plants and corals can actually use to photosynthesize. In plain terms, PAR measures the light your tank inhabitants can eat, not just the brightness your eyes perceive.
If you keep live plants or corals, PAR is the lighting number that matters most. A fixture can look dazzlingly bright and still deliver mediocre PAR, so learning to think in PAR rather than watts or lumens is the key to growing healthy photosynthetic life.
PAR vs lumens: why the difference matters
Lumens were invented to describe brightness for human vision, which is most sensitive to green and yellow light. Plants and corals do not share our eyes. They photosynthesize most efficiently using red and blue wavelengths, exactly the colors lumens tend to undervalue.
That mismatch means a light rated for high lumens can be a poor performer for growth, while a deep-blue reef light that looks dim to us can deliver excellent PAR for corals. PAR sidesteps the problem by counting all the usable photons in the photosynthetic range, regardless of how bright they appear to us.
| Metric | Measures | Useful for |
|---|---|---|
| Lumens | Brightness to the human eye | Room lighting, viewing |
| Watts | Power the fixture draws | Energy cost, not output quality |
| Kelvin | Color temperature | Appearance, not intensity |
| PAR (PPFD) | Usable light reaching a spot | Plant and coral growth |
You will often see PAR reported as PPFD, photosynthetic photon flux density, which is simply PAR measured at a specific point in micromoles per square meter per second. For practical purposes, when an aquarist says PAR they usually mean the PPFD reading at a given spot in the tank.
PAR targets for corals
Different corals evolved at different depths, so their light needs vary widely. Reading PAR at the spot where the coral sits lets you match each animal to a zone it will thrive in.
- Soft corals such as mushrooms and zoas: roughly 50 to 100 PAR.
- LPS corals such as hammers and torches: roughly 100 to 150 PAR.
- SPS corals such as acropora: roughly 200 to 400 PAR, near the top of the tank.
PAR drops fast as light travels through water, so a fixture delivering 300 PAR at the surface may give only 80 at the sand bed. Place demanding corals high and shade-loving ones low.
PAR targets for planted tanks
Planted tanks follow the same logic at lower numbers. The catch is that high light without matching CO2 and nutrients simply grows algae, so light is only one leg of the stool.
- Low light (java fern, anubias, mosses): about 15 to 30 PAR.
- Medium light (most stem plants, swords): about 30 to 50 PAR.
- High light (carpeting plants, red plants): 50 PAR and up, with CO2 injection.
If you are pushing into high-light territory, balance it with carbon by sizing your gas with our CO2 calculator. To estimate the right fixture for your tank dimensions and the livestock you keep, run our lighting calculator.
Measuring PAR in your tank
The only way to know your real PAR is to measure it, since manufacturer charts assume an exact mounting height and clean water. A PAR meter with a waterproof sensor lets you map readings at the surface, mid-water, and sand bed so you can place corals and set dimming levels with confidence. Many local reef clubs lend or rent meters if you only need to map a tank once.
PAR Meters for Mapping Your Tank
VABIRA IP68 PAR Meter for Aquarium and Reef
$129.80 on Amazon
Waterproof sensor on a telescopic rod, shows red, green, and blue PAR separately.
AquaHorti AH-Reef PAR Meter (PPFD and UVA)
$108.80 on Amazon
Reads PPFD plus UVA and color breakdown, with data logging for each spot.
Putting PAR to work
Once you think in PAR, lighting decisions get much simpler. Choose a fixture that can deliver your target PAR at your tank depth, then use a meter to verify and place livestock by their light needs. For help choosing a fixture, see our best LED aquarium lights roundup, and remember that on planted tanks light, CO2, and nutrients must rise together, not light alone.
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Frequently Asked Questions
What does PAR mean in aquariums?
PAR stands for photosynthetically active radiation. It measures the amount of light in the 400 to 700 nanometer range that plants and corals can actually use to photosynthesize. Unlike brightness ratings made for human eyes, PAR tells you how much usable light is reaching a given spot in your tank. It is the standard reefers and planted-tank keepers use to dial in lighting for healthy growth.
What is the difference between PAR and lumens?
Lumens measure brightness as the human eye perceives it, weighted heavily toward green and yellow light. Plants and corals do not see the world that way, and they rely heavily on red and blue wavelengths that lumens undervalue. PAR instead counts all the photons in the usable photosynthetic range. A light can look very bright in lumens yet deliver poor PAR for growth, which is why PAR is the better metric for tanks.
How much PAR do corals need?
It varies by coral type. Soft corals and many LPS corals do well in roughly 50 to 150 PAR, while light-hungry SPS corals often want 200 to 400 PAR or more near the top of the tank. PAR also falls off quickly with depth and distance from the light, so the reading at the sand bed is far lower than at the surface. Place corals at depths matching their PAR preference rather than guessing.
How much PAR do aquarium plants need?
Low-light plants like java fern and anubias thrive around 15 to 30 PAR, medium plants want roughly 30 to 50 PAR, and demanding carpeting plants often need 50 PAR or more along with CO2 injection. Pushing high PAR without enough CO2 and nutrients usually just feeds algae rather than plants. Match your light intensity to the plants you keep and the rest of your setup.
Do I need a PAR meter?
You do not need one to keep a tank, but a PAR meter takes the guesswork out of lighting, especially for expensive SPS corals or high-tech planted tanks. It lets you map exactly how much usable light reaches each spot so you can place livestock correctly and set your dimming levels. Many local reef clubs rent or lend meters, so you can map your tank once without buying one outright.
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