How Much Light Does Your Aquarium Need?
Enter your tank size and pick a light level for a lumens-and-watts ballpark — estimate-grade arithmetic from the fluorescent era, shown with its error bars and the PAR targets that modern planted-tank keeping actually relies on.
Lighting ballpark
Frequently asked questions
How many watts per gallon for a planted aquarium?
The classic fluorescent-era table runs about 1–2 WPG for low light, 2–3 for medium and 3–4 for high. It still gives a rough feel, but modern LEDs produce far more light per watt, so old WPG numbers usually overshoot and grow algae. Treat WPG as a legacy ruler: fine for an order of magnitude, wrong for buying a fixture.
How many lumens do aquarium plants need?
Common community figures put low light around 20–50 lumens per gallon, medium at 50–100 and high above 100 — but published sources disagree noticeably, which is the point. Lumens measure human-visible brightness, not plant-usable light, so two fixtures with identical lumen ratings can grow plants very differently.
What is PAR and why does it matter more than watts?
PAR — photosynthetically active radiation — counts the light wavelengths plants actually use, measured in µmol/m²/s at the substrate. Community targets run roughly 10–30 for low-light plants, 30–80 for medium and 80–120 or more for demanding carpets. Unlike watts or lumens, PAR is what your plants respond to, which is why serious fixture makers publish PAR maps.
What size light for a 20 gallon planted tank?
By the legacy rules, a 20 gallon medium-light tank wants roughly 1,000–2,000 lumens — about 40–60 old-fluorescent watts, or roughly 10–16 modern LED watts. By PAR, you want a fixture that delivers 30–80 µmol/m²/s at the substrate, which for a shallow 20 gallon is most mid-range clip-on plant lights. The PAR spec beats both conversions.
Is more light always better for plants?
No — light is the throttle on the whole system. More light drives faster growth, which then demands more CO2 and fertilizer; push light alone and algae is what takes up the slack. Most beginners get better results running medium light with stable CO2 and nutrients than maxing the fixture and fighting green water.
Do low-light plants need a special light?
Not really — mosses, Anubias, Java fern and most Cryptocoryne thrive under modest light, often the cheapest fixture on the shelf. Their failure mode is usually too much light burning leaves or feeding algae, not too little. If your stock is all low-light, buy for coverage and spectrum rather than raw power.
Does tank depth change how much light I need?
Dramatically — light intensity falls off with depth and with distance from the fixture, so a 20 gallon tall needs a stronger fixture than a 20 gallon long for the same substrate-level PAR. This is exactly the failure of gallons-based rules: they ignore geometry. A PAR meter reading, or the maker's PAR map at your depth, is the only reliable answer.
Why does the calculator refuse to give one exact number?
Because an exact number would be false precision. The inputs it knows — gallons and a light level — do not determine a unique wattage; depth, fixture efficiency, spectrum and mounting height all move the answer. Giving you a range with its confidence grade is the honest version of the calculation, which is more than most lighting calculators offer.
Sources & rule confidence
Every rule baked into this calculator traces to a community source and carries a confidence grade. A = multiple independent community confirmations; B = consistent direction, edge cases remain; C = direction only, numbers are reference values. We show disagreements instead of hiding them.
- R4 · C · reference value Lighting ballpark only: legacy fluorescent WPG low 1–2 / medium 2–3 / high 3–4; LED-era lumens per gal low 20–50 / medium 50–100 / high 100+; PAR at substrate low 10–30 / medium 30–80 / high 80–120 µmol/m²/s — sources disagree, trust PAR.
Honest C-grade: the watts-per-gallon table is a fluorescent-era ruler that runs wrong on modern LEDs, and published lumens-per-gallon figures disagree between sources (hygger puts medium at 50–100 lm/gal while aquariumbreeder puts it at 80–160). The disagreement is shown, not hidden. The reliable metric is PAR at the substrate — roughly 10–30 µmol/m²/s for low light, 30–80 for medium, 80–120 for demanding plants — so use this page for an order of magnitude and a fixture's published PAR map for the real number.
Disclaimer
All figures are estimates for freshwater community aquariums and do not replace the product label, a water test kit, or professional advice. When a rule on this page is graded C or marked estimate-grade, treat the numbers as an order of magnitude, not a target.