Planted Tank Lighting Guide

PAR matters more than watts

Plants don't care about watts — they care about PAR (photosynthetically active radiation), the amount of usable light reaching the leaves, measured in µmol/m²/s. Watts only measure power draw. The old "watts per gallon" rule was written for fluorescent tubes; a modern LED produces far more PAR per watt, so the rule overestimates LED needs.

As a rough map of PAR at the substrate:

• Low light: 15–30 — Anubias, Java fern, mosses, Cryptocoryne. Undemanding; no CO₂ needed.
• Medium light: 30–50 — most stem plants, swords, dwarf sag. CO₂ helps but isn't mandatory.
• High light: 50+ — carpeting plants (HC Cuba, dwarf hairgrass), red plants. CO₂ injection and regular fertilizer are required.

Until you own a PAR meter, LED watts-per-gallon rules of thumb still work as a starting point: 1–1.5 W/gal for low-tech, 2–3 W/gal for high-tech with CO₂.

Worked example: 20-gallon long, high-tech

• Dimensions: 30 × 12 × 12 in → 30 × 12 × 12 / 231 = 18.7 gallons
• High-tech target: 2–3 W/gal → 37–56 W of LED
• Pair it with: pressurized CO₂ at ~30 ppm, 8-hour photoperiod on a timer, and a complete fertilizer dosed weekly

Photoperiod: 6–10 hours, on a timer

Start new tanks at 6–8 hours and only extend once plants are established and algae-free. Longer light doesn't grow plants faster past their saturation point — it mostly grows algae. Consistency beats duration: plants adapt to a schedule, so use a cheap outlet timer rather than switching lights manually.

Spectrum: 6500K daylight

A full-spectrum "daylight" around 6500K grows plants well and looks natural to human eyes. Plants photosynthesize most efficiently in red and blue, but pure red/blue "blurple" grow lights make your tank look like a nightclub. Adjustable RGB or RGB+white fixtures let you tune the look without sacrificing growth. When comparing fixtures, check the CRI (color rendering index) too — 90+ renders fish and plant colors far more vividly than cheap 70-CRI lights.

Depth matters

PAR falls off with distance. A light that carpets a 12-inch-tall tank may barely grow moss in a 24-inch-tall one. Tall tanks need stronger fixtures (or accept low-light plants at the bottom); raising the light higher spreads coverage but lowers intensity.

Quick reference: light levels by plant type

How to judge your light without a PAR meter

Most hobbyists don't own a PAR meter. Three practical proxies work well:

• Manufacturer PAR charts. Reputable LED brands publish PAR readings at 12" and 24" depth — check before buying.
• Plant response. Leggy, upward-stretching stems and tiny new leaves mean too little light; thriving growth with algae on leaves means too much relative to CO₂.
• Phone lux apps (rough). Not PAR, but comparing readings before/after raising a light tells you how intensity changes with height.

The light–algae balance (the whole game)

Algae is rarely a "too much light" problem in isolation — it's a balance problem. Plants need light, CO₂ and nutrients in proportion. When light outruns the other two, algae — which needs almost nothing — moves in.

• High light + no CO₂ + lean ferts = algae farm. Either add CO₂ and fertilize properly, or dim the light.
• Fix algae by rebalancing, not by chemicals. Shorten the photoperiod or dim 20–30%, keep CO₂ stable through the whole photoperiod, dose ferts consistently. Algaecides treat the symptom.
• New tanks get diatoms and some algae regardless. It usually fades as plants establish (4–8 weeks). Don't redesign your lighting in week two.

5 lighting mistakes beginners make

• 12+ hour photoperiods "because the tank looks nice." Put viewing lights on a separate dim setting or enjoy the tank during its 8-hour window.
• No timer. Irregular light stresses plants and hands algae an opening.
• Upgrading light without CO₂/ferts. More light raises the ceiling — you must raise CO₂ and nutrients to match, or algae cashes in.
• Judging brightness by eye. Human eyes auto-adjust; a tank that "looks bright" can be 20 PAR. Trust the numbers, not your retinas.
• Blasting a new setup at 100%. Start dim or short, ramp up as plants root in.