Planted Tank Lights: PAR Over Watts for Real Plant Growth

Buy a planted tank light based on the PAR it delivers at your substrate, not on its wattage or lumen rating. Watts measure power draw. Lumens measure brightness the way your eye perceives it. Neither number tells you how much usable light reaches a leaf sitting near the bottom of the glass.

What PAR measures and lumens miss

PAR is photosynthetically active radiation, the 400 to 700 nanometre band plants can actually run photosynthesis on. It gets reported in micromoles per square metre per second, and it falls off sharply with distance.

Lumens fail here because the lumen curve is weighted to human vision, which peaks in green. Plants pull hardest on red and blue. A fixture can look blazing bright to you and still land mediocre PAR at the substrate.

Watts fail for a different reason. The old watts-per-gallon rule came out of the fluorescent era, when output per watt was consistent across tubes. LED efficiency and lens design vary so much between fixtures that wattage predicts almost nothing now.

Low-tech and high-tech PAR targets

Hobby convention splits substrate PAR into three bands: roughly under 30 is low light, 30 to 50 is medium, and above 50 is high. When a brand publishes a PAR chart, check its figures against your own tank depth.

Without injected CO2, stay in the low to medium band. Carbon runs out as the limiting factor long before light does, and every bit of intensity past that point feeds algae instead of plants. It is the most common reason people end up fighting algae in a planted tank.

With pressurised CO2 and a real dosing routine, high PAR is what colours up red stems and packs a carpet tight. It also punishes any gap in fertiliser within days.

Photoperiod does more work than intensity

Start at six hours a day on a timer and leave it there for a month. Plants settle into a consistent photoperiod, while algae takes advantage of an inconsistent one.

If growth looks clean after that month, add an hour. If you are scraping glass every week, cut intensity rather than hours, either by dimming the fixture or raising it off the rim.

Matching a fixture to tank depth

Depth is the variable most people underestimate. A light that reads 40 at 12 inches can deliver half that at 20 inches, so a tall tank needs a stronger fixture for the same plant list.

Dimming matters more than peak output. A dimmable unit lets you run low through the algae-prone first months, then climb as the tank matures. Check the spec sheet when you compare LED lights for planted tanks, because dimming is often buried in the listing.

Then match the light to the plants you want. Anubias, java fern and cryptocoryne finish fine at low PAR, which is why a modest fixture carries the low-tech route in our planted tank setup walkthrough.

Colour temperature and spectrum

Spectrum changes how a tank looks more than how it grows. Anything near 6500K renders greens and reds honestly, and full-spectrum white LEDs cover every common freshwater plant.

Heavy pink and purple grow-light spectra were built for greenhouse crops. They work, but they make fish look strange and they hide the early algae film you want to catch.