The short answer
For most freshwater tanks, run the light 6–8 hours a day on a timer. That’s enough for healthy plant growth and a pleasant viewing window, without handing algae the extra hours it needs to take over. The single best upgrade you can make is a timer so the schedule is consistent every day.
Why not longer?
It’s tempting to think more light means healthier plants — but plants can only use so much light per day. Beyond that point, the excess just feeds algae, which is far less fussy than your plants. Long or irregular lighting is one of the most common causes of green algae outbreaks.
A consistent, moderate schedule keeps plants happy and starves algae of the surplus. If you already have an algae problem, cutting the photoperiod back toward 6 hours is often the first fix.
Fine-tuning your schedule
- Low-tech / easy plants: 6–7 hours is plenty.
- High-tech / CO2 planted tanks: can push to 8 hours because plants use light faster when CO2 and nutrients are dosed.
- Fish-only tanks: light is just for viewing — keep it short to limit algae.
- Fighting algae? Drop to 6 hours and check the intensity isn’t too high.
Some keepers use a “siesta” — a few hours on, a midday break, then on again — which can help control algae while fitting your viewing times.
The bigger picture
Light is one leg of a triangle with CO2 and nutrients. If light outpaces the other two, algae fills the gap. Balance is everything.
Explore fixtures on the aquarium lighting hub. If algae is creeping in, see how to get rid of algae. For demanding plants, read do I need CO2 and our CO2 for beginners guide.