Light is the throttle on a planted tank. Everything else — CO2 demand, fertilizer demand, how fast algae appears — scales with how much of it you give the tank.
Intensity is the setting that matters
Brightness at the substrate, not wattage on the box, is what your plants respond to, and it falls off sharply with depth. The same fixture over a 12-inch-deep tank and a 24-inch-deep tank is two completely different light levels at the bottom. This is why carpeting plants fail in deep tanks under a light that grows them fine in a shallow one.
A dimmable fixture is worth paying for. It lets you start low, watch what happens, and raise the level as plant mass fills in — which is the correct order. Starting at full output over a sparsely planted new tank reliably produces algae, because there is plenty of light and almost nothing consuming it.
Spectrum: real, but oversold
Plants grow under a wide range of spectra, and most modern planted-tank LEDs are perfectly adequate. Spectrum's honest effect is on how the tank looks to you — a higher red and blue content makes reds and greens pop, which is why aquascaping fixtures are tuned the way they are. Choose a spectrum because you like how your fish and plants look under it. Do not expect it to fix a growth problem.
Photoperiod: shorter than you think
Six to eight hours is plenty for most planted tanks. Longer does not mean more growth — plants saturate, and the surplus goes to algae. Put the light on a timer and keep it consistent; plants respond badly to a schedule that drifts around your evenings. If you are fighting algae, cutting an hour off the photoperiod is the cheapest first move there is.
