Aquarium Photoperiod: How Long to Run Planted-Tank Lights

How long to run aquarium lights and why longer is not better. Learn typical photoperiods, the siesta idea, timers, and how duration differs from intensity.

Photoperiod is the number of hours your aquarium light runs each day. It is one of the easiest settings to control and one of the most misunderstood: longer is not better. Photoperiod (duration) is separate from intensity (how bright the light is), and the two must be set together with the plants' carbon and nutrient supply in mind. This guide covers how long to run the lights and why.

How long should the lights be on?

In a newly planted tank, start short. A common recommendation is about 6 to 8 hours a day, because the plants need time to adjust to their new surroundings, and a shorter cycle (such as 6 hours) is helpful for beginners who are worried about algae. As the plants grow in and the tank stabilises, the photoperiod can be increased, up to roughly 8 to 12 hours, and stable mature tanks can run 10 hours or more without adverse effects.

Why longer is not better

Photosynthesis only happens during the light window, and it only works when enough CO2 and nutrients are available. Once those run short, extra light hours do not buy more plant growth; the surplus light instead favours algae. Plants and algae use the same resources (light, nutrients and carbon dioxide), so the goal is to balance them so the plants outcompete the algae. This is why both too much and too little light can trigger algae.

The siesta (split photoperiod)

Some hobbyists split the photoperiod into two blocks with a dark 'siesta' in the middle, on the theory that a midday break lets CO2 recover and disrupts algae. It can be a reasonable experiment, but it is not a cure for an unbalanced tank: the underlying fix is matching light to the CO2 and nutrients available. A single, consistent block on a timer is simpler and works well for most setups.

Use a timer and keep it consistent

Put the light on a timer so it turns on and off at the same time every day. Timers are far more reliable than people: even if you are away or simply forget, the plants get a consistent schedule, which matters because stability is a key part of preventing algae. Many modern lights also offer a gradual sunrise and sunset ramp, which is fine but optional; consistency matters more than the ramp itself.

Finally, account for ambient light. A tank near a window receives extra daylight that adds to the effective photoperiod and can push an otherwise sensible schedule into algae territory, so factor that in when choosing duration.