PAR

PAR stands for photosynthetically active radiation, and it measures the amount of light in the specific wavelength range that plants can actually use for photosynthesis. It is a more useful number for planted tanks than lumens or watts, since those older measurements describe how bright a light looks to the human eye rather than how much usable energy reaches the plants.

PAR readings are taken with a dedicated meter and typically measured at the substrate, since light intensity drops significantly with water depth and can vary a lot across the footprint of a tank depending on the fixture and how it is mounted. Low-light plants and mosses can get by with modest PAR values, while demanding carpeting plants and high-light stem plants need substantially more, which is why matching plant choice to actual PAR at the substrate matters more than trusting a light's marketing claims.

Without access to a meter, aquarists often estimate PAR based on fixture wattage per gallon, mounting height and manufacturer charts, though these are rough substitutes at best. A common mistake is raising a light closer to the water to boost PAR without also considering CO2 and fertilizer levels, since more usable light without enough of the other two tends to push a tank toward algae rather than better plant growth.