How much PAR does a planted tank need?

2Hr Aquarist gives 20–40 µmol/m²/s at the substrate for low-light plants such as Anubias and Java fern, 40–60 for carpets and most common green plants, 60–90 for easier red plants, and 90–150 for high-demand stems, which usually also need CO2 injection.

How many hours of light for a planted tank?

2Hr Aquarist suggests 5–6 hours for a new tank (first 4–6 weeks) or a tank fighting algae, 7–8 hours for a stable low-tech tank and 8–9 hours for a stable CO2-injected tank. They call light beyond about 12 hours a day wasted.

Daily light integral is the total light delivered in a day, in moles of photons per square meter. It is PAR in µmol/m²/s times the hours of light times 3,600, divided by one million.

Planted Aquarium Light Calculator: PAR, Hours and DLI

2Hr Aquarist's PAR ranges at the substrate
PAR, µmol/m²/s	What grows
20–40	Low-light and shade plants: Anubias, Java fern, Bucephalandra, many Cryptocoryne
40–60	Carpeting plants and most common green plants
60–90	Easier red plants; more compact growth
90–150	High-demand stems and trickier species, usually with CO₂ injection; most red plants
150+	“Expert territory” where algae triggers bite hard

Where the ranges come from

The PAR ranges are 2Hr Aquarist's (2Hr Aquarist¹), measured at the substrate, which they call the only meaningful intensity number because that is where the plants live. They describe 200+ µmol as overkill for most setups. The photoperiods are from their guide to light hours (2Hr Aquarist²): 5–6 hours for a new tank or one fighting algae, 7–8 for a stable low-tech tank, 8–9 for a stable CO₂-injected tank, and no gain past about 12 hours. They also say a longer photoperiod can't make up for too little intensity.

Reading a PAR chart

Light is strongest straight below the center of the fixture and falls off with depth and toward the sides, by an amount that depends on the fixture's optics. Makers normally measure in a full tank (2Hr Aquarist³). Read the chart at your substrate depth, and expect the corners to get less than the center figure. Overlapping fixtures add their PAR together.

“Low” and “medium” aren't the same everywhere

Different makers use the same words for different numbers. Aquarium Co-op's Easy Green directions call 10–20 PAR low light and 20–35 PAR medium (Aquarium Co-op⁴), well below 2Hr Aquarist's bands. When a fertilizer label doses by light level, use its own definition.

DLI

The daily light integral adds up a day's light: PAR × 3,600 seconds per hour × hours ÷ 1,000,000, in mol/m²/day. The formula is from Virginia Cooperative Extension (Virginia Cooperative Extension⁵). It is a horticulture measure; aquarium guides mostly speak in PAR and hours, so use it to compare schedules, not as a target.

How much light the plants are getting

Where the ranges come from

Reading a PAR chart

“Low” and “medium” aren't the same everywhere

DLI

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