Choose the right aquarium light intensity for your tank.
Planted-tank lighting is specified in lumens per litre because what plants respond to is light delivered into the water column, not the wattage of the fixture. Roughly 25 lumens per litre supports low-light plants like anubias and java fern, 40 covers most stem plants, and 60 or more is needed for carpeting species — which at that level also require CO2 injection, since light without carbon becomes an algae problem rather than a growth benefit. Converting lumens to LED wattage uses an efficacy of about 80 lumens per watt, typical of aquarium LED fixtures. Water depth matters too: light attenuates with depth, so a tall tank needs more output at the surface to deliver the same intensity at the substrate.
Aquarium Lighting
Volume gallons = length x width x depth / 231; litres = gallons x 3.78541; lumens = litres x lumens per litre; LED watts = lumens / 80
It was a fluorescent-era rule and no longer works, because LED efficacy varies by a factor of three between fixtures. Lumens or PAR per litre is the meaningful measure.
Because light drives growth only if carbon and nutrients keep pace. High light with no CO2 injection leaves plants unable to use it, and algae — which are far less demanding — take the surplus.