Aquarium lighting, measured properly.

Lighting is where the planted-tank hobby gets unnecessarily confusing, because fixtures are marketed in units that have little to do with what plants actually use. A light advertised by its lumens or wattage tells you how bright it looks to you, not how much photosynthetic energy reaches your plants. The unit that matters is PAR, and the relationship between light, CO₂, and nutrients is what separates a thriving planted tank from an algae-covered one. This guide explains how aquarium lighting really works. The lighting calculator converts between PAR, lux, and lumens and recommends fixtures by light level.

PAR, Lux, and Lumens — What Each Measures

These three units get used interchangeably and should not be. Lumens measure brightness as perceived by the human eye, weighted toward green light because that is what our eyes are most sensitive to. Lux is lumens spread over an area — brightness per square meter. Neither tells you what plants receive, because plants do not photosynthesize with human-weighted brightness; they use specific wavelengths, especially in the red and blue ranges.

PAR — Photosynthetically Active Radiation — measures the light in the 400–700 nm range that plants actually use for photosynthesis, as photon flux at a given point. PAR is the only one of the three that predicts plant growth, which is why serious planted-tank keepers talk in PAR and largely ignore lumens. The catch is that PAR falls off with depth and distance, so the PAR at your substrate is much lower than at the surface — making tank depth, from the tank volume dimensions, a real factor. The lighting calculator helps translate the lumens/lux a fixture advertises into a usable PAR estimate.

The key insight: plants respond to PAR, not the lumens or watts on the box. A "bright" light can deliver low PAR, and a modest-looking light can deliver plenty. Always think in PAR at the substrate, not brightness at the surface.

Low, Medium, and High Light

Planted tanks are grouped by the PAR reaching the substrate, and the level you choose dictates everything else about the setup. Low light (roughly 15–30 PAR) grows hardy, undemanding plants — anubias, java fern, cryptocoryne, mosses — with no CO₂ injection and minimal fuss; it is the forgiving, beginner-friendly tier. Medium light (30–50 PAR) opens up more species and faster growth but usually wants CO₂ supplementation and more attention to nutrients. High light (50+ PAR) grows demanding carpeting plants and vivid colors, but it is a demanding regime that requires pressurized CO₂ and a tight nutrient balance — and punishes any imbalance with algae.

The crucial point is that more light is not better. Light is the accelerator of the whole planted system, and pushing it without matching CO₂ and nutrients does not grow better plants — it grows algae. Most successful planted tanks live happily in the low-to-medium range.

The Light–CO₂–Nutrient Triangle

Plant growth depends on three inputs in balance: light, CO₂, and nutrients. Light drives the rate of photosynthesis; CO₂ and nutrients are the raw materials photosynthesis consumes. When all three are matched, plants thrive and outcompete algae. When light outpaces the CO₂ and nutrients available, the excess light energy has nowhere to go in the plants — and algae, which are far less demanding, exploit it. This is the single most important concept in planted-tank lighting: algae is usually a sign of light out of balance with CO₂ and nutrients, not of "too much light" alone.

This is why throwing a powerful light at a struggling planted tank backfires. The fix for poor plant growth is rarely just more light; it is balancing the triangle — often adding CO₂ or fertilizer to match the light you already have, or reducing light and photoperiod to match the CO₂ you can supply. The lighting calculator flags the algae risk implied by your light level and CO₂ situation.

Photoperiod and Practical Setup

How long the light runs matters as much as how strong it is. A photoperiod of 6–8 hours suits most planted tanks; longer does not mean more growth and often means more algae, especially in tanks without CO₂. A consistent schedule on a timer beats erratic lighting. If algae appears, shortening the photoperiod and reducing intensity are the first, cheapest levers — before adding chemicals. Floating plants and shade-tolerant species let you run lower light successfully. And remember that lighting interacts with the rest of the tank: strong light raises plant CO₂ demand, which interacts with surface agitation from your filter (which off-gasses CO₂), and healthy plants in turn support water quality. None of these levers work in isolation.

Choosing a Light Without a PAR Meter

Most hobbyists never own a PAR meter, so the practical question is how to choose a fixture without one. Start from your plants, not the light: decide whether you want a low-, medium-, or high-light setup, because that dictates everything downstream. For a low-light, no-CO₂ tank — the right choice for most people — a modest, reputable LED rated for your tank's footprint is plenty, and you can dial it back with height or dimming if algae appears. For medium and high light, look for fixtures that publish PAR-at-depth charts for tank depths like yours, since a manufacturer willing to publish PAR is telling you something useful that lumens never will. Tank depth is the quiet variable: PAR falls off sharply with distance, so a tall tank needs a stronger fixture to deliver the same substrate PAR as a shallow one of equal length, which ties light selection to the dimensions from your tank volume measurement.

When in doubt, choose less light and a shorter photoperiod, then increase deliberately. It is far easier to add light to a healthy low-light tank than to claw back an algae outbreak in an over-lit one. The lighting calculator helps translate the lumen and lux figures fixtures advertise into a usable light-level estimate so you can shop with the right number in mind.

Acclimating Plants and Avoiding the Early Algae Phase

New planted tanks almost always pass through an algae-prone phase in the first weeks, before the plants establish and start outcompeting it — and aggressive lighting makes it worse. The standard fix is to start conservative: run a shorter photoperiod (six hours or less) and modest intensity for the first month while plants root and grow, then ramp up gradually as the plant mass increases and can use the extra light. Dense initial planting helps enormously, because a tank packed with growing plants from day one leaves fewer nutrients and less light for algae. Floating plants provide instant shade and soak up excess nutrients during this vulnerable window. Patience through the establishment phase, with light kept in check, is what separates a tank that matures into lush growth from one that stalls under a green film.

Frequently Asked Questions

What is PAR and why does it matter more than lumens?

PAR (Photosynthetically Active Radiation) measures the light wavelengths plants actually use for photosynthesis, while lumens and lux measure brightness as the human eye perceives it. Plants grow in response to PAR, not lumens, so PAR is the only unit that predicts plant growth.

How much light does a planted aquarium need?

It depends on your plants. Low light (about 15–30 PAR at the substrate) grows hardy plants with no CO₂; medium light (30–50 PAR) and high light (50+ PAR) grow more demanding species but require CO₂ and careful nutrient balance. Most tanks do well in the low-to-medium range.

Why do I have algae in my planted tank?

Usually because light is out of balance with CO₂ and nutrients. Excess light energy that the plants cannot use — for lack of CO₂ or fertilizer — feeds algae. The fix is to balance the three, often by reducing light and photoperiod or adding CO₂, rather than simply adding more light.

How long should I run my aquarium lights?

Six to eight hours a day suits most planted tanks. Longer photoperiods rarely improve growth and often encourage algae, especially without CO₂. Use a timer for consistency, and shorten the photoperiod as a first response to algae.