Aquarium Light Duration Calculator

Why Aquarium Light Duration Matters

Light duration, or photoperiod, is one of the most important parts of an aquarium lighting schedule. It defines how many hours per day your tank is illuminated, combining both natural daylight from windows and artificial light from your fixtures.

Fish, invertebrates, aquatic plants, and corals all evolved under regular day–night cycles. When we keep them in indoor aquariums, we must reproduce those cycles as closely as is practical. A well-chosen photoperiod helps:

• Support healthy plant growth and coral photosynthesis
• Limit nuisance algae fueled by excessive light
• Provide predictable day–night cues that reduce stress in fish
• Align feeding and activity patterns with a stable routine

Too little light can stunt growth, weaken plants, and cause corals to lose color. Too much light can drive algal blooms, aggravate existing algae problems, and leave fish with very little true “night,” which they need to rest.

How This Aquarium Light Duration Calculator Works

This calculator estimates a daily artificial light duration based on two things:

1. Your tank type (freshwater planted, freshwater fish-only, or reef tank)
2. The average number of natural daylight hours that reach the aquarium

For each tank type, there is a typical recommended total photoperiod range:

• Freshwater planted tank: usually 8–10 hours of total light per day
• Freshwater fish-only tank: usually 6–8 hours of total light per day
• Reef tank with corals: usually 10–12 hours of total light per day

The calculator starts with a representative target photoperiod within these ranges and then subtracts the natural daylight your aquarium already receives. The result is the suggested number of hours to run your aquarium lights so that the combined natural and artificial light fall into an appropriate range.

Basic Formula Behind the Recommendation

The underlying idea is simple. We choose a target total daily light duration based on tank type, then reduce that by the contribution from natural light:

Where:

• L = recommended hours per day of artificial light
• T = target total daily light duration (photoperiod) for your tank type
• D = hours per day of natural daylight reaching the aquarium

If the computed value of L is very low or negative, it means your tank already receives as much or more light from the room than is typically advisable for that tank type. In that situation, you may want to reduce window exposure, add blinds, or move the aquarium rather than adding more artificial light.

How to Use the Aquarium Light Duration Calculator

1. Select your tank type. Choose whether you keep a freshwater planted tank, a freshwater fish-only tank, or a reef tank with corals.
2. Estimate natural daylight hours. Count how many hours per day the aquarium receives direct sun or bright indirect daylight from windows or skylights. Use an average over a typical day in the current season.
3. Enter the daylight value. Type this number into the Natural Daylight Hours field. Using decimals is fine (for example, 4.5 hours).
4. Run the calculation. Click the button to calculate your suggested artificial light duration.
5. Adjust your timer. Set your lighting timer or controller to run your aquarium lights for approximately the recommended number of hours per day.

After you change your lighting schedule, give your aquarium at least 1–2 weeks before making another big adjustment so you can observe how plants, corals, and algae respond.

Interpreting Your Results

The calculator output is a starting point, not a rigid rule. Use it to set a reasonable initial lighting schedule and then refine it based on what you see in your tank.

Signs your photoperiod may be too long

• Rapid growth of green, brown, or filamentous algae on glass, rocks, or plants
• Cloudy water with visible algal bloom despite normal maintenance
• Fish appearing restless, rarely experiencing dark conditions
• Corals showing fading colors despite otherwise stable water parameters

If you see these signs, reduce artificial light duration by 30 minutes to 1 hour per day and monitor for two weeks.

Signs your photoperiod may be too short

• Slow or halted growth of otherwise healthy plants or corals
• Plants with yellowing leaves or corals with reduced extension that are not explained by nutrients or water chemistry
• Fish that appear sluggish only during the short lit period and hide excessively

In that case, increase light duration by 30 minutes and reassess after 1–2 weeks, avoiding sudden jumps of several hours at once.

Typical Light Duration by Tank Type

The table below summarizes common target ranges for total daily light and the approximate artificial light duration you might expect if your aquarium receives a few hours of natural daylight.

Your specific recommendation from the calculator may differ slightly from these simple examples, depending on the natural daylight value you enter and the exact baseline used for each tank type.

Worked Example: Freshwater Planted Aquarium Near a Window

Imagine you have a 29-gallon freshwater planted aquarium next to a bright but not sun-blasted window. You estimate that the tank receives about 3.5 hours per day of bright indirect daylight.

For a typical planted tank, you might aim for a total daily photoperiod of about 9 hours. Using the simple formula above:

The calculator would recommend running your aquarium lights for roughly 5.5 hours per day. In practice, you would likely round to a convenient setting such as 5.5 to 6 hours on a timer.

Next, you would watch for:

• Plant health: Are leaves pearling and growing steadily without turning pale?
• Algae: Are algae levels stable or decreasing over the next few weeks?
• Fish behavior: Do fish have a clear day and night and behave calmly?

If algae remains low and plants appear healthy, the schedule is likely appropriate. If plants seem weak but algae is still under control, you might increase the artificial photoperiod to around 6.5 hours and reassess.

Refining Your Aquarium Lighting Schedule Over Time

The recommended duration from this calculator is intended as a starting point. Each aquarium is unique, and you should refine your schedule gradually based on observation.

Make changes slowly

• Limit changes to about 30–60 minutes at a time.
• Allow at least 1–2 weeks between major changes so your tank has time to respond.
• Avoid sudden shifts from very long to very short photoperiods (or the reverse), as they can stress sensitive species.

Use a timer or controller

Manual switching is inconvenient and inconsistent. A basic plug-in timer or programmable controller ensures:

• Lights turn on and off at the same time every day
• Fish and invertebrates experience a predictable routine
• You are less tempted to leave the lights on “just a bit longer”

Assumptions and Limitations of This Calculator

This calculator focuses on light duration, not every detail of aquarium lighting. Keep these assumptions and limitations in mind:

• Light intensity is not calculated. The tool does not estimate PAR, lumen output, or spectrum. Two tanks with the same duration but very different light fixtures may see different results.
• Tank depth and aquascape are simplified. Deeper tanks, very dense planting, or heavy rockwork can reduce how much light reaches lower areas, even with the same schedule.
• Species-specific needs vary. Some delicate plants and corals need more or less light than the general ranges used here. Always cross-check with care guides for your particular species.
• Natural daylight is an estimate. The field for natural daylight hours assumes that your estimate of sun or bright indirect light is reasonably accurate. Large seasonal changes or very cloudy climates can alter real exposure.
• Existing algae issues are not fully addressed. If your tank already has a severe algae problem, simply changing the light duration may not be enough. Nutrients, stocking, and maintenance also play major roles.
• No account for dawn/dusk staging. If you use ramp-up or ramp-down modes (dimmer light at the start and end of the day), this calculator treats the total lit time as a single block for simplicity.

Because of these limitations, treat the recommended duration as an informed guideline rather than a strict prescription. Combine it with good husbandry practices such as regular water changes, appropriate feeding, and careful stocking.

When to Deviate From the Suggested Duration

There are times when you may want to intentionally differ from the calculator output:

• High-light aquascapes: Advanced planted tanks with pressurized CO₂ and high-intensity LEDs may safely run somewhat longer photoperiods, provided algae is controlled and nutrients are balanced.
• Recovery periods: During algae outbreaks or after major disturbances, temporarily shortening the photoperiod can help stabilize the system.
• Very low-light fish-only tanks: If your primary goal is showcasing fish and you have no live plants, you may choose a shorter daily light window for aesthetic reasons.
• Seasonal simulation: Some aquarists purposely vary the photoperiod through the year to simulate seasons for breeding projects or natural behavior.

In all of these cases, change durations stepwise and keep monitoring your livestock closely.

Summary

The Aquarium Light Duration Calculator gives you a practical starting point for setting your aquarium lighting schedule. By combining your tank type with the natural daylight your aquarium already receives, it estimates how long to run your artificial lights each day to reach a reasonable total photoperiod. Use the result as a guide, observe how your plants, corals, and fish respond, and make small adjustments over time while keeping the assumptions and limitations above in mind.