How Much Light Does a Planted Aquarium Need?

Lighting is where a lot of planted tanks quietly go wrong. Too little and the plants thin out and melt; too much, without matching nutrients and carbon dioxide, and you grow a beautiful crop of algae instead. Getting the light right is less about buying the brightest fixture and more about matching the intensity to the plants you want to keep.

This guide gives you a workable target in lumens, explains the difference between low, medium and high light, and shows how to avoid the most common lighting mistake. It also covers why lumens are only a starting point and what actually drives plant growth.

The quick answer

A practical guide is to size your light by brightness relative to volume, in lumens per litre:

• Low light plants: about 20 lumens per litre
• Medium light plants: about 40 lumens per litre
• High light plants: 60 or more lumens per litre, usually with added carbon dioxide

For a 100 litre tank that is roughly 2,000 lumens for a low-light setup, 4,000 for medium and 6,000 or more for high light. To turn your own tank into a target figure, the aquarium light calculator does the maths once you enter your volume and planting level. Not sure of the volume? Measure it first with the fish tank volume calculator.

Low, medium and high light explained

The right level depends entirely on the plants you want, not on how bright you would like the tank to look.

Light level	Lumens per litre	Plants it suits	Needs CO2?
Low	15 to 25	Java fern, Anubias, mosses, cryptocoryne	No
Medium	25 to 50	Most stem plants, swords, vallisneria	Optional
High	50+	Carpeting plants, red plants, demanding species	Usually yes

The safest place to start, especially for a first planted tank, is low to medium light with easy plants. It grows steadily, forgives mistakes and rarely triggers algae. You can always add light later; it is much harder to walk back an algae outbreak.

Why lumens are only a starting point

Lumens measure brightness as the human eye sees it, but plants respond to a different thing called PAR, the portion of light they can actually use for photosynthesis, and to the light’s spectrum. Two fixtures with the same lumens can grow plants very differently depending on their PAR and colour output.

So treat the lumens-per-litre figure as a sensible starting target, then adjust based on how your plants actually grow. If lower plants stretch upward and go leggy, they want more light. If you see algae on the glass and leaves, you likely have too much light for the available nutrients and carbon dioxide.

The mistake that causes algae

The single most common planted-tank mistake is running high light without matching carbon dioxide and nutrients. Light drives growth, but plants also need carbon and food to use it. When light outpaces the other two, algae, which are far less fussy, take the surplus instead. The result is a bright tank covered in green.

The fix is balance, not just less light. Either lower the intensity and photoperiod to match a low-tech setup, or commit to added carbon dioxide and fertiliser to support the higher light. Both work; mismatching them is what fails.

Photoperiod: how long, not just how bright

Most planted tanks do well with 6 to 8 hours of light per day. Longer does not mean more growth, it usually means more algae, because plants have a daily limit on what they can process. A timer keeps it consistent, and consistency matters more than squeezing in extra hours. If you battle algae, shortening the photoperiod is often more effective than reducing intensity.

Depth matters too

Light fades as it travels down through water, so a tall tank loses more of it before reaching the substrate than a shallow one of the same volume. A deep tank, or one aimed at carpeting plants on the bottom, needs a stronger light than the litres alone suggest. When in doubt on a tall tank, size up a level and use the light calculator as your baseline.

Frequently asked questions

How many lumens per litre for aquarium plants? About 20 lumens per litre for low-light plants, 40 for medium and 60 or more for high-light plants. Enter your volume in the light calculator for a total in lumens.

How many hours should aquarium lights be on? Six to eight hours a day suits most planted tanks. Longer photoperiods tend to feed algae rather than plants, so use a timer for consistency.

Why is my planted tank getting algae? Usually too much light for the available carbon dioxide and nutrients. Lower the intensity or photoperiod, or add carbon dioxide and fertiliser to match the light.

Do all aquarium plants need CO2? No. Low and many medium light plants grow well without added carbon dioxide. Only high light and demanding carpeting plants usually require it.

Does a deeper tank need a stronger light? Yes. Light weakens with depth, so a tall tank needs a brighter fixture than a shallow one of the same volume to reach plants near the substrate.

The bottom line

Match the light to the plants, not the other way round. Pick a light level from the table, get a lumen target from your volume with the fish tank volume calculator and the aquarium light calculator, and keep the photoperiod to 6 to 8 hours. Balance light with carbon dioxide and nutrients, and you grow plants instead of algae.