Aquarium plant lighting is the single biggest factor in what you can grow. Match the light to the plants rather than the tank: undemanding ferns and mosses do well in low light, while carpets and red stem plants need considerably more. Get this wrong and no amount of fertiliser rescues it.
How much light your plants need
Tropica, which grows and classifies most of the plants sold in Europe, scales light to the same Easy, Medium and Advanced categories it uses for plant difficulty. Its guidance is that if you have chosen nothing but Easy plants, 10 to 20 lumen (0.25 to 0.5 watts) per litre is adequate. For Medium plants it recommends 20 to 40 lumen (0.5 to 1 watts) per litre, while Advanced plants require more than 40 lumens (1 watt) per litre.
Treat those as bands, not targets. They are useful because they line up with how plants are actually sold, so you can read the difficulty label on a pot and know roughly what you are committing to before you buy.
Shade plants and sun plants
Aquatic plants split fairly neatly into two groups, and the split explains most beginner disappointments.
- Shade plants: ferns and Cryptocoryne grow healthily in low light, and some of them actively do worse under high intensity. Anubias, Java fern and Cryptocoryne wendtii belong here.
- Sun plants: stem plants photosynthesise strongly and grow faster under intense light. Rotala and the red-leaved species sit here, and they need the light to colour properly.
More light is not simply better. Pushing a shade plant under a strong fixture is a reliable way to grow algae on it instead.
Light has to reach the floor
This is where carpet plants catch people out. A carpet only closes if strong light reaches all the way down to the substrate and stays there. Starve it and the plant stretches upward hunting for light, thins out, and never knits together.
Depth matters more than most people expect, which is part of why a wider, shallower tank is easier to plant than a tall one.
Light, CO2 and nutrients move together
Raising light raises demand for everything else. Strong light over a tank without matching CO2 and nutrients does not give you faster plant growth, it gives you algae, because the plants cannot use the light they are being given.
That is the reasoning behind the standard beginner advice to keep light moderate at first. A modest fixture over hardy plants is far more likely to succeed than a powerful one over a sparse new tank. The Walstad method takes this to its logical end and runs deliberately low-tech throughout.
Lighting questions
How many lumens per litre do aquarium plants need?
Tropica’s guidance is 10 to 20 lumen per litre for Easy plants, 20 to 40 for Medium, and more than 40 for Advanced. Check the difficulty rating on the plants you are buying and work back from that.
Can you have too much light in a planted tank?
Easily. Light drives demand for carbon and nutrients, so a strong fixture over a tank that cannot meet that demand favours algae over plants. Some shade species also grow poorly under high intensity regardless of what else you supply.
What about PAR?
PAR measures the light plants can actually use for photosynthesis, so it describes what reaches a given point in the tank better than a lumen figure does. It is also hard to measure without a meter, which is why lumens per litre and plant difficulty ratings remain the practical way most aquascapers choose a light.