Aquarium lighting marketing loves to talk about wattage, but wattage tells you almost nothing about whether a light will actually grow plants. Three other measurements matter far more, and understanding them will save you from the two most common lighting mistakes: too little light for growth, or too much light relative to your CO2 and nutrient dosing.
PAR: The Number That Actually Matters
Photosynthetically Active Radiation (PAR) measures the intensity of light in the specific wavelength range plants actually use for photosynthesis. Low-light plants like Anubias and Java Fern thrive at 15-30 PAR, while carpeting plants and high-tech setups often need 50-100+ PAR at the substrate. Two fixtures with identical wattage can deliver wildly different PAR depending on their LED efficiency and reflector design — this is why comparing lights by watts alone is close to meaningless.
Kelvin and Color Temperature
Color temperature, measured in Kelvin, describes the visual warmth or coolness of the light rather than its growing power. Most planted tanks look best and grow well in the 6,500K-8,000K range, which balances a crisp, natural-looking white with enough spectrum in the red and blue bands that plants use most efficiently. Very high Kelvin values (10,000K+) skew heavily blue and can look striking on a reef tank but often under-deliver the red spectrum planted tanks need.
Photoperiod: Consistency Over Duration
How long your lights run matters less than how consistently you run them. Six to eight hours of stable, uninterrupted light will nearly always outperform ten inconsistent hours, and a longer photoperiod without matching CO2 and nutrients is the single most common cause of an algae outbreak in an otherwise healthy tank. If you are fighting algae, cutting photoperiod by an hour is often a faster fix than changing anything else in your routine.
Matching Light to Your Actual Setup
A CO2-injected, heavily-dosed tank can support high PAR and a longer photoperiod because the plants can actually use that extra light. A low-tech, non-CO2 tank should stay conservative on both fronts — in a low-tech setup, more light without more CO2 simply hands the advantage to algae, which are far less picky about their growing conditions than your plants are.