PAR in Aquarium Lighting: What It Means and Why It Matters More Than Wattage
PAR stands for Photosynthetically Active Radiation. It measures the amount of usable light, specifically the wavelengths plants and photosynthetic corals can actually use for photosynthesis, reaching a given point in the tank, rather than just how bright a light looks to the human eye. A PAR meter reports this in micromoles per square meter per second (µmol/m²/s), a unit describing how many particles of usable light hit a spot on the substrate or coral each second. Two fixtures can look equally bright and still deliver very different PAR readings at the same depth, which is why PAR, not wattage, is the number to compare.
How PAR Is Measured
A PAR meter, sometimes called a quantum meter, uses a sensor that reads only the photosynthetically useful wavelengths rather than the full visible spectrum a lux meter measures. You hold the sensor at the point in the tank that matters, usually at substrate level for a planted tank or at coral placement depth for a reef, and read the µmol/m²/s value directly. PAR varies a great deal by location in the tank: a reading near the surface directly under the fixture and another near the glass at the substrate can differ by a factor of five or more under the same light. Most hobbyists without their own meter rely on manufacturer PAR charts, which map expected readings at different depths and mounting heights for a specific fixture, though those charts reflect the manufacturer’s own test tank and can vary from setup to setup depending on water clarity and mounting height.
Why PAR Matters More Than Wattage
Wattage tells you how much electricity a fixture draws, not how much usable light reaches your corals or plants. LED efficiency, lens angle, spectrum, and mounting height all change how much PAR actually lands in the tank, so two lights of identical wattage can produce very different results at the same depth. Low-light plants such as anubias and java fern, and soft corals such as mushrooms and zoanthids, generally do fine on modest PAR in the 20 to 75 µmol/m²/s range. SPS corals and high-light carpeting plants such as dwarf hairgrass or Monte Carlo need substantially more, often 150 to 400 µmol/m²/s depending on species and tank depth.
What Happens When PAR Is Wrong
Running PAR too low stalls growth. Plants stretch toward the light source and lose color, and corals close up, fade, or slowly starve rather than dying outright. Running PAR too high causes different problems depending on the tank. In a reef tank, corals not acclimated to the intensity can bleach, expelling the symbiotic algae they depend on for food and color. In a planted tank, light in excess of what CO2 and nutrients can support is one of the most common triggers for algae blooms, since the plants cannot use all the energy the light supplies and algae fills the gap instead. A CO2-injected high-tech planted tank generally needs more light than a low-tech one for exactly this reason. See our guide to CO2 & Bubble Count for how the two are linked.
Why Depth Changes the Answer
PAR drops significantly with water depth as light scatters and is absorbed on the way down, so the same fixture at the same mounting height delivers less usable light to the substrate in a deep tank than in a shallow one. A fixture that comfortably supports SPS corals in a 30 cm deep frag tank may fall well short of the same corals’ needs in a 60 cm display tank, even mounted at the same height above the waterline. This is the most common reason a livestock choice that worked in one tank underperforms in another, and it is why PAR charts always specify the depth or mounting height they were measured at.
FAQ
What is PAR in aquarium lighting?
PAR (Photosynthetically Active Radiation) is the amount of light in the wavelengths plants and corals can use for photosynthesis, measured in µmol/m²/s at a specific point in the tank. It describes usable light output, not overall brightness.
What is a good PAR level for coral?
It depends on what you are keeping. Soft corals generally do well at 50 to 100 µmol/m²/s, LPS corals around 100 to 200, and SPS corals typically need 200 to 400 or more, always measured at the coral’s actual placement depth rather than at the surface.
How much PAR is too much?
Too much PAR for a given coral or plant shows up as bleaching or tissue recession in corals, and pale new growth or algae outbreaks in plants. The threshold varies by species and by how gradually the livestock was acclimated to the light, so a sudden jump in intensity is riskier than a high steady-state level reached slowly.
Does higher wattage mean higher PAR?
Not reliably. Two fixtures with the same wattage can produce very different PAR depending on LED efficiency, lens design, and spectrum, which is exactly why PAR rather than wattage is the number to compare when choosing a light.
Do I need a PAR meter to keep corals or plants successfully?
No, but it removes a lot of guesswork. Most hobbyists get by using a fixture’s published PAR chart and adjusting height or intensity based on how the livestock actually responds, which works for straightforward setups but is less precise than a direct reading.
Shop Aquarium Lighting
See Marine Aquarium Lights for reef fixtures or Planted Aquarium Lights for freshwater planted setups. For a deeper look at a specific PAR-capable fixture and how to mount it, see our EcoTech Marine Reef Lighting Guide.