LED spectrum: what plants use vs what looks good

PAR, PUR, Kelvin, and why a bright-looking light can still grow algae instead of plants. How to read an LED spectrum chart and pick a light that does both jobs.

Aquarium LEDs produce light. Plants use some of that light for photosynthesis and ignore the rest. Algae use a slightly different slice. The gap between "light that looks bright to your eyes" and "light that plants can use" is where most confusion lives.

PAR, PUR, and why Kelvin is mostly useless

PAR (Photosynthetically Active Radiation) measures the total number of photons hitting a surface per second in the 400-700 nm wavelength range. This is the full visible spectrum from violet to deep red. PAR is measured in micromoles per square meter per second. Low-light plants need 15-30 PAR at the substrate. Medium-light plants need 30-50. High-light demanding species need 50-100+. The lighting calculator gives PAR targets by plant type.

PUR (Photosynthetically Usable Radiation) is the subset of PAR that chlorophyll actually absorbs efficiently. Chlorophyll a has absorption peaks around 430 nm (blue) and 660 nm (red). Chlorophyll b peaks around 453 nm (blue) and 642 nm (red). Light in the green-yellow range (500-580 nm) is poorly absorbed by chlorophyll, which is why plants look green: they reflect that wavelength back at your eyes.

A light can have high PAR but low PUR if most of its output is in the green-yellow range. Your eyes see it as very bright. Your plants barely register it. This is why "watts" and "lumens" are poor indicators of plant growth. Lumens measure perceived brightness, which peaks at 555 nm (green), right where plant absorption is weakest.

Kelvin (color temperature) describes the overall appearance of white light: warm (3000K, yellowish), neutral (5000K), daylight (6500K, bluish-white), or cold (10000K+, very blue). It tells you how the light looks to you. It tells you almost nothing about the spectral distribution at the wavelengths that matter for plants. Two lights rated at 6500K can have very different emission spectra depending on the phosphor blend used.

What spectrum grows plants

For freshwater planted tanks, you want strong output in two ranges:

Blue: 420-460 nm. Drives chlorophyll a and b absorption. Promotes compact leaf growth, strong cell development, and good coloration in red plants. Blue light penetrates water better than red, making it important for deeper tanks.

Red: 630-670 nm. The primary driver of photosynthesis. Chlorophyll a absorbs most strongly around 660 nm. Red light is efficient at driving the photosynthetic reaction centers, but it attenuates faster in water than blue. In tanks deeper than 45 cm (18 in), red light reaching the substrate drops off significantly.

Green: 500-580 nm. Not useless, despite the common claim. Green light penetrates leaf canopies better than red or blue because upper leaves let most of it pass through. In densely planted tanks, green light reaches lower leaves that red and blue can't. It also contributes to photosynthesis via carotenoid pigments and some absorption by chlorophyll b. But its efficiency per photon is lower than blue or red.

A good planted tank light has peaks in the blue and red with enough white/green to make the tank look natural to human eyes. Full-spectrum white LEDs (the kind made from a blue diode with a yellow phosphor coating) produce a broad curve with a blue spike and a long tail through green, yellow, and into red. They work. They're not optimal, but they work well enough for most setups.

RGB LEDs (separate red, green, and blue diodes) allow precise control over the spectrum. High-end planted tank lights like the Chihiros WRGB or Twinstar use this approach. You can increase the red channel for better plant growth and dial back the green to reduce the "washed out bright" look that high-green-output lights produce.

Why cheap LEDs grow algae

A common frustration: someone buys a budget LED strip, cranks it up, and gets algae instead of plant growth. The light is bright. The tank looks well-lit. But the plants aren't growing fast enough to outcompete algae.

Several things might be happening:

High PAR in the wrong spectrum. If the LED puts most of its output in the green-yellow range, plants can't use it efficiently. Algae, particularly green algae and cyanobacteria, have different pigments (phycocyanin, phycoerythrin) that absorb across a broader spectrum including the green-yellow range. They can use light that plants can't.

Too much light without CO2. High PAR without supplemental CO2 creates a bottleneck. Plants can only photosynthesize as fast as the limiting factor allows. If CO2 is the limit, extra light doesn't speed up plant growth; it just feeds algae that can photosynthesize at lower CO2 concentrations. In tanks without CO2 injection, keeping PAR below 30-40 at the substrate prevents this imbalance.

Photoperiod too long. Running lights for 12+ hours doesn't compensate for low intensity. It just extends the window for algae to grow. 6-8 hours of appropriate-intensity light with the right spectrum grows plants better than 14 hours of the wrong light.

Reading a spectrum chart

Any decent LED manufacturer publishes a spectral power distribution (SPD) chart. The x-axis is wavelength in nanometers (400-700). The y-axis is relative intensity. Look for:

A peak around 440-460 nm (blue). This should be one of the strongest features on the chart.
A peak or at least meaningful output around 630-660 nm (red). Budget white LEDs often have very little red because the phosphor tails off. This is the most common deficiency.
A broad hump through 500-600 nm (green-yellow). Some is fine. If this is the dominant feature and the red peak is tiny, the light will look bright but underperform for plants.

You don't need lab equipment to evaluate this. Just find the SPD chart on the product page. If the manufacturer doesn't publish one, that's a red flag; they're probably selling a generic LED strip with an unoptimized spectrum and no interest in the planted tank market.

Practical recommendations

For low-tech tanks (no CO2, easy plants): any full-spectrum white LED rated at 6500K with 15-30 PAR at substrate depth works. Don't overthink it. Anubias, java fern, cryptocoryne, and java moss grow under almost any light.

For medium-tech tanks (optional CO2, moderate plants): look for an LED with documented PAR readings at your tank depth, a published SPD showing distinct blue and red peaks, and adjustable intensity. Run it at 30-50 PAR. Nicrew, Fluval Plant 3.0, and Hygger full-spectrum are all in this range.

For high-tech tanks (injected CO2, demanding plants, red coloration): invest in an RGB or WRGB fixture with channel control. Run 50-100 PAR at the substrate. Increase the red channel for color in rotala, ludwigia, and alternanthera. Balance with CO2 injection or you'll grow algae. The CO2 calculator helps keep the light-CO2 relationship in check.