Aquarium lighting duration: the one setting that stops algae
The right aquarium lighting duration sits between 6 and 8 hours a day, uninterrupted and at fixed times thanks to a 5-15 € timer. It’s the number-one lever against algae: in the vast majority of green tanks, the light stays on 12 to 14 hours, from getting up to going to bed. Cutting the duration costs zero euros and solves one invasion in two.
Why does too much light feed algae?
Plants and algae compete over the same equation: light + CO2 + nutrients. Higher plants saturate their photosynthesis after 8 to 10 hours; beyond that, the surplus light benefits only the algae, which are more opportunistic. The same logic applies to natural light: a tank facing a window gets uncontrolled “lighting” that turns the water green, sometimes to the point of needing a UV sterilizer.
How many hours for your tank?
- New tank (first 2 months): 6 hours, while the plants root in and gain the upper hand.
- Balanced planted tank: 8 hours, extendable to 9-10 if the plants grow well without algae.
- Lightly planted or goldfish tank: 6 to 7 hours are plenty — the fish don’t need more.
- Algae outbreak in progress: drop back to 6 hours for 3 to 4 weeks, but never run a prolonged blackout, which exhausts the plants too.
Is a timer really essential?
Yes — it’s the best value-for-money accessory in the hobby: 5 to 10 € for a mechanical one, 10 to 20 € for digital or smart versions. Consistency matters as much as duration: erratic schedules stress fish and plants alike. Set the window around your hours at home, say 12 pm to 8 pm: the aquarium lights up for your evenings, not for the cat. Recent LED fixtures add gradual sunrise and sunset effects — pleasant but optional; find all our equipment guides on the fishkeeping hub.
Should you use a midday break?
The “siesta” (4 hours of light, 2 to 4 hours of darkness, then 4 hours of light) lets CO2 build back up at midday and disrupts the algae’s cycle. Reports are mixed: try it if 6 continuous hours aren’t enough, but fix the basics first — overfeeding, high nitrates, too few plants. Lighting is only one side of the triangle, along with the nutrients and CO2 covered in our article on fertilizer and CO2.
Frequently asked questions
Does room light count towards the duration?
No, ambient lighting is too weak for photosynthesis. Only the tank’s own light and direct sun on the glass count.
Do fish need total darkness at night?
Yes, they have a genuine sleep cycle. Avoid permanent blue “moonlight” LEDs, which also encourage certain algae.
Is reducing intensity better than reducing duration?
Both work. If your fixture is dimmable, 50 to 70 % intensity over 8 hours is often the best plant-algae compromise.
This guide is part of Planète Pets’s Fishkeeping universe. Our advice is general in nature: for any health concern, your veterinarian remains the only reference.