The short answer
Yes — more light is one of the biggest algae triggers, but with an important caveat: light causes algae when there’s more of it than your plants and CO2 can actually use. Light itself isn’t the enemy; light out of balance with nutrients and CO2 is. Both how long the lights are on and how bright they are feed algae when plants can’t keep up with the surplus.
Why light drives algae
Algae and plants both need light to grow. In a balanced tank, healthy plants absorb the light and nutrients first, leaving little for algae. But push the light beyond what your plants can use — because it’s too bright, on too long, or your plants are struggling — and algae takes the excess energy. This is why the most common algae fix is simply turning the light down or shortening the day.
- Photoperiod (hours): long light periods, especially over 8–10 hours, give algae ample time to grow. Keep it to 6–8 hours on a timer.
- Intensity (brightness): a fixture that’s too strong for a low-tech tank floods it with more light than the plants can process.
- Sunlight: direct sun on the glass is uncontrolled, intense light — a frequent cause of green water and stubborn algae.
Getting light in balance
The goal is light your plants can fully use, and no more.
- Use a timer. Consistent, controlled hours matter more than anything. Set 6–8 hours and stick to it.
- Match light to plants and CO2. If you run strong light, support it with fertilisers and stable CO2 so plants can keep up. See our planted-tank light picks.
- Dim or raise the fixture if plants are fine but algae persists on a bright setup.
The bottom line
Light causes algae only when it outruns your plants. Control the hours with a timer, match intensity to your setup, and keep sunlight off the glass. For more, see how long lights should be on to avoid algae and browse the aquarium lighting hub.