Most planted tanks do best on 6–8 hours of light per day. Start at 6 hours, run it at the same time every day using a timer, and adjust only in 30-minute increments based on what you observe. Consistency matters more than the exact number of hours.
The instinct when setting up a planted tank is to maximize light time — more light means more growth, which seems logical. In practice, the relationship between light hours and plant growth is not linear. Beyond a threshold (roughly 8 hours for most setups), additional hours stop accelerating plant growth and start feeding algae instead.
Algae and plants compete for the same resources — light, CO₂ and nutrients. The difference is that algae responds faster to increases in light availability and can establish itself before plants grow into the available energy. Six hours is a safe baseline. It gives aquatic plants enough photosynthesis time to grow steadily while keeping the light window narrow enough that algae stays suppressed in most setups. From there, you adjust based on evidence.
The specific hours of the day are less important than the total duration and consistency. A planted tank light on a timer — same start time, same end time, every day — gives plants a predictable photoperiod and removes the most common cause of light-related problems: irregular schedules that create windows algae can exploit.
A workable starting schedule:
Running lights in the afternoon keeps the tank visually active during hours you are most likely to be home. It also avoids morning sunlight from nearby windows adding uncontrolled light that extends your effective photoperiod beyond the timer setting. If you work from home or want the tank lit earlier, a 10:00 AM to 4:00 PM window works equally well. What matters is picking a window and keeping it consistent, every day, without gaps.
Once a baseline schedule is running, the only reason to change it is direct evidence — not theory. Two observations tell you an adjustment is needed:
One variable at a time. Changing light duration and fertilization simultaneously makes it impossible to identify which change produced the result — or the problem. Patience here is the actual skill; most light-schedule issues correct themselves within two weeks of a single adjustment.
The siesta method splits the photoperiod into two windows with a gap in the middle: for example, 8:00 AM to 12:00 PM, off for two hours, then 2:00 PM to 6:00 PM. The theory is that the midday break depletes CO₂ that algae was absorbing during the first window, and the second light period finds plants better positioned to use the remaining carbon in the water.
In practice, the benefit is real but modest. If you run CO₂ injection and are seeing green spot algae or green dust algae despite careful nutrient management, a siesta is worth testing. For tanks without CO₂ injection, a standard uninterrupted photoperiod is easier to manage and produces comparable results in most setups. Start simple — complexity can always be added later if a specific problem requires it.
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