The short answer
Keep your aquarium lights on for 6 to 8 hours a day, run through a timer for consistency. That range gives plants what they need while denying algae the extra hours it thrives on. Longer photoperiods — especially over 10 hours, or leaving lights on all day — are one of the most common algae triggers. If you’re fighting algae, shortening the day to around 6 hours is the quickest, cheapest fix to try first.
Why 6–8 hours
Algae and plants both feed on light. In a balanced tank, plants use the light first — but the longer the lights are on, the more surplus light there is for algae to grab once the plants have had their fill.
- Under 6 hours: plants may not get enough, and can struggle over time.
- 6–8 hours: the sweet spot for most freshwater tanks — enough for plants, not enough to fuel algae.
- Over 8–10 hours: increasingly favours algae, particularly if plant growth or CO2 can’t keep up.
Whatever length you pick, consistency matters most — a fixed daily schedule beats an erratic one.
Use a timer, and mind sunlight
The easiest way to hold a steady photoperiod is to take yourself out of the loop.
- Put the light on a timer so it’s the same every day without you remembering. See our planted-tank light picks and the aquarium lighting hub.
- Keep sunlight off the tank — daylight streaming onto the glass adds uncontrolled hours of intense light that a timer can’t manage, and is a classic algae cause.
Light is only part of it
Photoperiod is the biggest single lever, but it works alongside feeding and nutrients — too much of either still feeds algae even on a perfect schedule. For the full picture, see does more light cause algae and why you have so much algae.