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Planted Tank Photoperiod Calculator – Prevent Algae

Aquarium Plants aquarium calculator photoperiod
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Low: Anubias, Ferns, Mosses
High: Needs CO₂ & fertilizers
Pressurized or DIY CO₂ system
% of substrate covered by plants
New tanks need shorter photoperiods
Current algae presence in tank
When lights turn on each day
Recommended Photoperiod
Risk: Moderate
7.0 hours / day

Standard Continuous Schedule

8:00 AM 3:00 PM 7h

Siesta Method Schedule

8:00 AM 11:00 AM 2h Break 1:00 PM 5:00 PM

Dark break helps suppress algae while plants recover quickly.

Algae Prevention Guide
Quick Best Practices
  • Use a timer for consistent lighting every day
  • Start with 6 hours for new planted tanks
  • Increase photoperiod gradually (30 min/week)
  • Perform weekly water changes (30-50%)
  • Clean filters and remove dead leaves regularly
  • Consider a siesta break if algae appears

Frequently Asked Questions

A photoperiod is the duration of time your aquarium lights are on each day. In planted tanks, this typically ranges from 6 to 10 hours. The photoperiod directly influences plant photosynthesis, algae growth, and the overall health of your aquatic ecosystem. Consistent daily photoperiods are essential — irregular lighting schedules stress plants and encourage algae outbreaks.
Algae are highly opportunistic and can outcompete plants under prolonged or inconsistent lighting. When the photoperiod is too long (typically over 10 hours), plants reach their photosynthetic saturation point while algae continue to grow unchecked. Additionally, fluctuating CO₂ levels during extended photoperiods create unstable conditions that favor algae. A properly calculated photoperiod ensures plants have enough light to thrive without giving algae an advantage.
The Siesta Method splits your photoperiod into two lighting blocks separated by a 2-4 hour dark period in the middle of the day. The science behind it: algae require continuous light to maintain efficient photosynthesis, while higher plants can rapidly restart photosynthesis after a dark break thanks to stored enzymes. When lights come back on, plants resume within minutes, but algae struggle to recover. This method is especially effective against green dust algae and hair algae. Many aquarists report significant algae reduction after implementing a siesta schedule.
The ideal photoperiod depends on several factors: light intensity, CO₂ availability, plant density, and tank maturity. As a general guideline: low-light tanks without CO₂ can run 7-9 hours, medium-light tanks with CO₂ do well at 7-8 hours, and high-light tanks with CO₂ injection typically need 6-7 hours. New tanks should start at just 5-6 hours and gradually increase. Use our calculator above for a personalized recommendation based on your specific setup.
Yes — CO₂ injection enables plants to utilize light more efficiently, which means you can safely extend the photoperiod by 1-2 additional hours compared to a non-CO₂ tank at the same light intensity. However, CO₂ must be stable and consistent throughout the photoperiod. It's recommended to start CO₂ injection 1 hour before lights on and stop it 1 hour before lights off to maintain stable levels. Fluctuating CO₂ is worse than no CO₂ at all and can trigger severe algae outbreaks like BBA (Black Beard Algae).
Newly established tanks (under 1 month) have immature biological filtration and unstable nutrient cycles. Plants are still adapting to submerged growth, and their root systems aren't fully developed. A shorter photoperiod of 5-6 hours reduces the metabolic demand on plants and limits excess nutrients that would otherwise fuel algae. As the tank matures and plants establish, you can gradually increase the photoperiod by 30 minutes each week until reaching the target duration.
Look for these warning signs: ① Green dust algae on glass within 2-3 days of cleaning, ② Hair algae or thread algae growing on plant leaves, ③ Plants showing signs of stress like yellowing or transparent leaves, ④ Excessive pearling early in the photoperiod followed by sluggish behavior, and ⑤ Cyanobacteria (blue-green algae) appearing on substrate. If you notice any of these, reduce your photoperiod by 1 hour and observe for 1-2 weeks.
Yes, many aquarists split their photoperiod to enjoy their tank both in the morning and evening. A common split schedule is: 4 hours in the morning (e.g., 7:00-11:00 AM), a 5-6 hour dark break during the workday, and 4 hours in the evening (e.g., 5:00-9:00 PM). This is essentially an extended siesta method. Just ensure the total light duration stays within the recommended range and the dark break is at least 3-4 hours to effectively suppress algae.
Different algae types signal different imbalances: Green Dust Algae (GDA) on glass often indicates excessive light duration. Hair/Thread Algae suggests too much light relative to CO₂ levels. Blue-Green Algae (Cyanobacteria) can appear when photoperiod is too long combined with low nitrates. Staghorn Algae frequently indicates light/CO₂ imbalance. Reducing the photoperiod is always the first and safest intervention before adjusting fertilizers or CO₂.
Densely planted tanks have more plants competing for light and nutrients, which means they can often handle a slightly longer photoperiod (by about 30-60 minutes) because the plants collectively outcompete algae for resources. Sparsely planted tanks have fewer plants to absorb nutrients, leaving more available for algae — these tanks benefit from shorter photoperiods. As your plants grow and fill in, you can gradually extend the lighting duration.
Pro Tips for Algae-Free Planted Tanks

Consistency is everything. Plants adapt to stable conditions. Use a quality timer, avoid random light schedule changes, and always dim or reduce photoperiod after major tank maintenance. Observe your plants — they'll tell you if the light is right. Healthy plants with vibrant colors and steady growth indicate a well-balanced photoperiod. If in doubt, shorter is safer than longer when it comes to lighting.

Happy Plants, Happy Fish

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