FishBaseHQ
Glass aquarium with green plants under aquarium lighting

Equipment · 8 min read

Aquarium light schedule

Six to eight hours on a timer. All day is how the glass goes green.

How long aquarium lights should stay on for fish-only and planted tanks, when to ramp a new setup, and how to cut algae without killing plants.

Photo: Unsplash

Lights are not just so you can see the fish. They drive algae and plants. A timer beats “I forgot and left them on until midnight.”

TankHoursNotes
Fish-only, plastic plant6–8Enough to look at. Less algae pressure.
Low-tech planted6–8Do not make up for a weak light by running it longer.
High-tech + CO₂~8, often rampedIntensity and CO₂ matter more than a 12-hour day.

Size the fixture with lighting, then keep the clock boring.

New tanks

Start at 5–6 hours for two weeks. Add 30–60 minutes a week if plants are growing and algae is not. A brand-new scape under 10 hours of strong LED is an algae kit.

Intensity vs hours

A dim bar for 10 hours can be gentler than a blazing bar for 6. Both grow algae if you overfeed and skip plants. High light without fertilizer and CO₂ is how you get green water. Algae. CO₂. Fertilizer.

Split photoperiod (optional)

Four hours on, two off, four on. Some people use it against green dust algae. Not required. A single 7-hour block on a timer is the default.

Fish need dark

24/7 lighting stresses them. Room light at night is not a rave, but a spotlight all night is. Give them a predictable off period.

If algae explodes: shorten by 1–2 hours for two weeks before you buy another bottle. Wipe the glass after you change the clock, not instead of it.

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