LED Planted Aquarium Lighting Guide | Match Light To Tank

Match a full-spectrum LED aquarium light to tank width and depth, then run it 8–10 hours daily — that keeps most planted tanks healthy.

Get LED planted aquarium lighting wrong and the tank shows it fast — algae wins, stems stretch, or leaves melt. The fix is choosing a full-spectrum light with enough intensity for your depth and putting it on a steady schedule.

Three numbers decide the outcome: PAR, spectrum, and photoperiod. This guide walks through each one with real specs from Fluval and NICREW fixtures, so the next light you buy is the one the tank actually needs — not the one with the longest product description.

What A Planted Tank Light Needs To Do

A planted tank light has one job: deliver enough photosynthetically active radiation (PAR) at the substrate for the plants you keep. Spectrum matters second, and how long the light runs each day matters third.

PAR measures the light plants can actually use, in µmol/m²/s — and water eats light fast. That steep falloff is why matching a fixture to tank depth matters more than matching it to tank length.

Full-spectrum 6500K white is the standard for planted tanks because it lands close to midday sunlight. Fixtures push photosynthesis further with dedicated color channels —

Light doesn’t work alone. Crank a high-output fixture without adjusting CO2 and fertilizer, and the tank grows algae faster than plants — surplus PAR has to go somewhere. Low-tech tanks do best with moderate light and 8–10 hour days; high-tech tanks with injected CO2 can push both intensity and photoperiod further.

What PAR Does A Planted Tank Need?

Low-light plants such as Anubias, Java fern, and moss grow happily at 20–40 PAR. Medium-demand stems and most crypts want 40–80, and high-light carpeting plants or red stems need 80 or more at the substrate.

A deeper tank needs a stronger unit or a second light bar, and the lux figures tell the same story: 24,000 lux at 3 in drops to 4,330 at 12 in.

Matching The LED To Your Tank

Match a fixture to the tank’s width class first, then verify depth, voltage, and water resistance — in that order.

• Width comes first. Length alone under-specs a light. Fluval’s Plant 3.0 covers 15–24 in tanks; the Plant 4.0 line reaches 48–60 in; NICREW makes five SlimLED sizes (N18921–N18925) spanning 12–18 in up to 48–54 in.
• Depth sets your plant choices. If the substrate sits below the fixture’s strong PAR range, drop to low-light plants or add a second bar.
• Voltage is regional.
• Water resistance varies. An IP43 fixture needs to sit well above the water line.
• Run it on a timer. Eight to ten hours a day is the working range; longer stretches feed algae as much as plants.

Fluval publishes the full measurement table — PAR and lux readings at 3, 6, and 12 inches — on the official Plant 3.0 specifications page, worth a check against your tank’s actual depth.

Prices shift between sellers.

The mistakes that bite most buyers: sizing by tank length alone, assuming a fixture works on any mains voltage, and skipping a timer so the light runs too long and algae takes over. All three are easy to avoid before checkout.

If you’d rather skip the math, our roundup of the best aquarium lighting for planted tanks compares the top fixtures side by side.

The formula that works: pick the size class that matches tank width, confirm the PAR reaches the substrate for the plants you chose, check voltage and IP rating, then run the light 8–10 hours a day on a timer. Consistency beats brightness — give the tank a few weeks before changing anything.