PAR

Definition

PAR (Photosynthetically Active Radiation) measures the amount of light in the 400-700 nm wavelength range available for photosynthesis. In reef tanks, PAR is the make-or-break parameter for coral and host-anemone health, expressed in micromoles per square meter per second (µmol/m²/s).

Practical use in aquariums

SPS coral and host anemones generally need 250-500+ PAR at the placement; LPS thrive at 100-250 PAR; soft corals tolerate 50-150 PAR. Measured with a quantum meter (Apogee MQ-510 or equivalent) - reef LEDs at the manufacturer-stated wattage do NOT guarantee target PAR; depth, water clarity, and reflector geometry all matter.

How PAR fits the bigger picture

Understanding PAR matters because it's connected to broader husbandry decisions: lighting drives photosynthesis, coloration, and growth - get it wrong and corals/anemones bleach or recess.

Browse the Fast Aquatics care library for full husbandry tutorials covering PAR in context.