The 6-8 Hour Rule
6 hours for starter tanks or algae-prone setups. 7 hours for established balanced tanks. 8 hours max unless you see specific deficiency signs.
Use a timer — never leave to manual on/off. Consistent schedule is as important as duration.
Why Not More?
Plants saturate on CO2 within a few hours in low-tech (no injection). More light after saturation just fuels algae.
Algae doesn't need CO2 for growth — it will use nitrate and phosphate with extended light.
Siesta Method
Alternative: 4 hours on, 3 hours off, 4 hours on. Plants get total 8 hours but algae struggles with interrupted cycle.
Works well for tanks where you see afternoon algae bloom. Not necessary for all tanks.
Intensity Guide
30-40 PAR at substrate level for low-tech. This is about 20W LED over a 40cm tall tank (rough guide).
Higher PAR = needs CO2 injection or plants will crash. Keep intensity moderate for low-tech success.