Fish care calculator
Aquarium Light Duration Calculator | FishZone
Dial in the perfect photoperiod for your aquarium. Enter your tank type, plant density, and viewing schedule to calculate the optimal daily light duration that fuels plant growth, suppresses nuisance algae, and gives your fish a healthy day/night cycle — without guesswork.
- Scientific Formula
- Reviewed by Experts
- Updated May 2026
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Type
Risk Assessment
Inputs
3
Difficulty
Beginner
Calculation Time
Instant
Quick Answer
The number one cause of aquarium algae is too much light. A common beginner mistake is leaving the aquarium light on for 12–14 hours a day to 'see the fish.' However, most planted aquariums only need 6 to 8 hours of light per day for optimal photosynthesis. If you do not have live plants, your tank only needs light when you are actively viewing it — ambient room light is enough for the fish. To enjoy your tank in the morning and evening without causing algae, use a 'Siesta Schedule' with a plug-in timer: 4 hours on in the morning, 4 hours off during the afternoon, and 4 hours on in the evening.
Enter your details
Enter the labeled values below. Results appear without leaving this page.
How to Use This Calculator (Worked Example)
This tool requires 3 key inputs:
- Biological Setup: Enter your specific value (e.g., from your tank's test kit or dimensions).
- Light Fixture Intensity: Enter your specific value (e.g., from your tank's test kit or dimensions).
- Current Algae Status: Enter your specific value (e.g., from your tank's test kit or dimensions).
Example: If you input average baseline values, the calculator will immediately process the formula and return the recommended output and safety warnings above.
Method
This tool uses the visible inputs on the page and returns practical aquatics guidance for maintaining your tank.
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The Problem
An aquarium owner is struggling with persistent algae despite regular cleaning, or has plants that are failing to grow despite having what appears to be adequate light. They do not understand the relationship between light duration, CO2 availability, and nutrient balance — and are either running their lights for far too long (the most common mistake) or not giving their plants a consistent enough schedule for healthy growth. They need a clear, actionable photoperiod recommendation based on their specific tank type and goals, without needing to understand the underlying biology in detail.
When to Use This Tool
Use this calculator when setting up a new aquarium and establishing a lighting schedule for the first time, when an existing tank develops a persistent algae problem that has not responded to cleaning, when plants begin melting or growing poorly despite appearing to have adequate light, when you want to switch to a siesta schedule to fit your viewing times around a work schedule, or when you have made changes to your CO2 or fertiliser regime and need to recalibrate your photoperiod to match.
Who is this for?
- Beginner aquarium keepers who are experiencing their first algae outbreak and don't understand the role their lighting schedule is playing
- Planted tank hobbyists trying to find the right balance between enough light for plant growth and too much light that triggers algae
- Fish-only tank owners who want to understand how much light their fish actually need for health, behaviour, and natural circadian rhythm
- Aquascapers setting up a new high-tech planted tank who need to dial in a photoperiod that works with their CO2 injection and fertiliser regime
- Reef aquarium keepers managing coral photoperiods, actinic ramp schedules, and PAR levels across a mixed coral system
- Hobbyists recovering from a severe algae outbreak — green water, hair algae, or cyanobacteria — who want to reset their tank's light schedule correctly
- Anyone who has been running their aquarium lights without a timer and wants to establish a consistent, healthy photoperiod for their fish and plants
Light is the engine that drives everything in an aquarium. If you provide too little light, your live plants will melt and die. If you provide too much light, your tank will turn into a green, algae-filled swamp within weeks. Getting the photoperiod right is one of the most impactful and most overlooked adjustments a fishkeeper can make — and it costs nothing but a timer.
Who Should Use This Calculator?
This tool is useful for any aquarium owner who has live plants struggling to grow, persistent algae that won't respond to cleaning, fish showing signs of stress from irregular light cycles, or a new tank they want to set up correctly from the start. It is particularly essential for:
- Planted tank and aquascape hobbyists trying to balance plant growth without triggering algae outbreaks
- Beginners who have been leaving their lights on all day and are now dealing with green water or hair algae
- Fish-only tank owners who want to know how much light their fish actually need for health and natural behaviour
- Reef aquarium keepers managing coral photoperiods and the transition from blue actinic to white spectrum throughout the day
- Anyone cycling a new tank who wants to know whether to run lights during the cycling process
The Photosynthesis Window
Aquatic plants have a maximum daily capacity for photosynthesis. Once they reach this limit — usually around 6 to 8 hours under moderate to high intensity light — they stop absorbing light and nutrients. However, algae never stops. If you leave your lights on for 12 hours, the plants use the first 6 to 8 hours productively, and algae uses the remaining time to grow unchallenged. This is why duration matters as much as intensity: a dim light left on for 14 hours will cause more algae than a bright light run for 7 hours.
The relationship between light, CO2, and nutrients is also critical. In a tank without CO2 injection, plants are often carbon-limited — they cannot photosynthesise efficiently even with abundant light. Running lights for long durations in a low-tech tank without CO2 is one of the most reliable ways to trigger a severe algae outbreak, because the excess light energy has nowhere productive to go.
What is a Siesta Schedule?
A 'Siesta' is a deliberate mid-day break in your lighting period — for example, lights on from 8 AM to 12 PM, off from 12 PM to 4 PM, then on again from 4 PM to 8 PM. This approach offers two significant advantages:
- It allows you to view your tank before work in the morning and after work in the evening without exceeding the 8-hour daily maximum that keeps algae in check.
- It disrupts the photosynthetic cycle of primitive single-celled algae. Higher-order vascular plants (like stem plants, anubias, and mosses) can adapt to and recover from a mid-day dark period. Single-celled algae — including the organisms that cause green water and green dust algae — struggle to resume their metabolic cycle after a complete blackout, giving your plants a significant competitive advantage.
Light Duration by Tank Type
Not all aquariums have the same lighting needs. As a practical starting guide:
- Fish-only tank (no live plants): 6–8 hours. Fish need a consistent day/night cycle for biological rhythm and immune health, but no specific spectrum or intensity is required. Lights should be off for at least 8 hours every night.
- Low-tech planted tank (no CO2 injection): 6–7 hours at low-to-moderate intensity. More than this without CO2 supplementation almost always leads to algae. Start at 6 hours and increase only if plants show deficiency symptoms.
- High-tech planted tank (CO2 injection): 8–10 hours at moderate-to-high intensity. CO2 injection gives plants the carbon they need to use more light productively, allowing longer photoperiods without triggering algae — but only when CO2, nutrients, and light are in balance.
- Reef / saltwater coral tank: 8–12 hours total, typically structured as a gradual ramp-up through blue actinic spectrum in the morning, peak white/full-spectrum midday, and a gradual ramp-down in the evening. Coral photoperiod management is more nuanced than freshwater and depends heavily on coral species and light intensity (PAR).
- New cycling tank: Minimal or no lighting until the nitrogen cycle is complete. Running full photoperiods during cycling frequently results in algae blooms that are difficult to eliminate later.
The Right Way to Fix an Algae Problem with Light
If you are already dealing with algae and want to use photoperiod adjustment to help, do not simply turn off your lights for days at random. A structured approach works significantly better. First, reduce your current photoperiod by 2 hours and hold that schedule for two weeks while monitoring the algae response. If the algae is improving, maintain the shorter schedule. If you have a severe outbreak of green water or cyanobacteria, a full 3-to-4-day blackout (complete darkness, tank fully covered) is the most effective reset — follow it with a shorter, structured photoperiod thereafter. Photoperiod adjustment alone rarely solves algae completely; it must be paired with addressing the nutrient imbalance that allowed the algae to establish in the first place.
Why You Should Always Use a Timer
Consistency is as important as duration. Fish and plants are highly sensitive to circadian rhythm disruption — irregular light schedules cause measurable stress in fish, suppress plant growth, and create the environmental instability that opportunistic algae exploits. A simple plug-in mechanical timer costs very little and will do more for your tank's health than almost any other single investment. Set it once, and the tank runs on a consistent biological schedule whether you are home or not.
Reference Table
Static Aquarium Light Duration & Photoperiod Calculator Reference
These baseline ratios and decision rules are included directly in the page so visitors can review core guidance before using the interactive calculator.
| Scenario | Baseline Rule | Safety Note |
|---|---|---|
| Inputs used by this tool | Biological Setup, Light Fixture Intensity, Current Algae Status | The interactive calculator refines the result from these inputs. |
| Outputs generated | Hours, Schedule | The static table gives baseline logic; final value depends on entered values. |
| Tank Type | Recommended Photoperiod | Light Intensity (PAR) | CO2 Required? | Siesta Schedule Recommended? | Primary Algae Risk | Timer Essential? |
|---|---|---|---|---|---|---|
| Fish-only (no plants) | 6–8 hours | Low (any — for viewing only) | No | Optional | Minimal if photoperiod controlled | Yes |
| Low-tech planted (ambient CO2) | 6–7 hours | Low–Medium (15–40 PAR) | No | Yes — highly recommended | Hair algae, green spot algae | Yes — critical |
| Medium-tech planted (liquid carbon) | 7–8 hours | Medium (30–60 PAR) | Liquid carbon only | Recommended | Green dust algae, hair algae | Yes |
| High-tech planted (CO2 injection) | 8–10 hours | Medium–High (50–150+ PAR) | Yes — pressurised | Optional | BBA if CO2 drops, hair algae | Yes — critical |
| Aquascape (iwagumi / Dutch) | 8–9 hours | High (80–150+ PAR) | Yes — pressurised | Optional | BBA, green spot algae | Yes — critical |
| Coldwater / Goldfish tank | 8–10 hours | Medium (30–60 PAR) | No | Optional | Green water, filamentous algae | Yes |
| Reef / Marine coral tank | 8–12 hours (ramped) | Very High (100–300+ PAR at depth) | No (but calcium/alkalinity) | No — gradual ramp preferred | Dinoflagellates, hair algae | Yes — smart controller recommended |
| New cycling tank (no fish yet) | 0–4 hours max | Minimal | No | No | Green water, diatoms | Yes — keep minimal |
| Brackish tank | 8–10 hours | Low–Medium (20–50 PAR) | No | Optional | Diatoms, filamentous algae | Yes |
| Shrimp tank (planted) | 6–8 hours | Low–Medium (15–40 PAR) | Optional | Yes — recommended | Hair algae, staghorn algae | Yes |
Troubleshooting Guide
1 I reduced my photoperiod but still have algae after two weeks.
Possible cause: Photoperiod reduction alone rarely eliminates an established algae colony immediately — algae that is already attached to surfaces has stored energy reserves and will persist for some time after light reduction. The more likely issue is that the tank has a co-existing nutrient imbalance (excess phosphate or nitrate), an uncontrolled external light source (window), or that the reduction in hours was not sufficient for the current CO2 and nutrient levels.
First, check for any external light sources — cover the sides and back of the tank if near a window. Test phosphate and nitrate levels; if phosphate is above 1 ppm or nitrate is above 20–30 ppm, increase water change frequency to bring levels down. Introduce fast-growing stem plants (Hornwort, Hygrophila polysperma, Water Wisteria) which will outcompete algae for available nutrients. Physically remove as much algae as possible manually to reduce the colony size. If the algae is specifically on glass, an algae scraper used every 2–3 days during the correction phase will keep it from re-establishing while the underlying conditions improve.
2 My plants are melting or turning yellow after I reduced my photoperiod.
Possible cause: A photoperiod reduction of more than 1–2 hours at once can cause a deficiency shock in plants that have adapted to higher light levels. Alternatively, if the plants were already marginal in health — due to nutrient deficiency or CO2 limitation — a light reduction may have pushed them below their minimum functional threshold.
Reduce photoperiod more gradually — no more than 30 minutes per week. Check that you are dosing a complete fertiliser that includes nitrogen, phosphorus, potassium, and micronutrients (iron is particularly important for plant colour). If the tank has no CO2 injection, consider adding liquid carbon supplementation (Seachem Excel or similar) to help plants make more efficient use of lower light. Remove any heavily melting leaves promptly to prevent them from decomposing and spiking ammonia.
3 Green water (pea soup) returned within a week of completing the blackout treatment.
Possible cause: The blackout eliminated the visible bloom but did not address the underlying cause — excess nutrients and/or too-long a photoperiod. Restarting the same photoperiod after the blackout recreated the exact conditions that caused the original bloom.
After any blackout treatment, restart at no more than 6 hours of lighting per day. Perform a 50% water change before restarting lights to reduce dissolved nutrients. Clean the filter thoroughly, as dead algae accumulates in the filter media and can release nutrients as it decomposes. Consider adding a UV steriliser — UV sterilisers are extremely effective at permanently eliminating free-floating green water algae by destroying the cells as water passes through the unit. Maintain the reduced photoperiod for at least 4 weeks before making any upward adjustments.
4 I am using a timer but my light comes on at random times or flickers.
Possible cause: Mechanical plug-in timers can drift over time, particularly cheap models, and may also be affected by power outages that reset the timer position. Flickering is usually caused by an incompatibility between the dimming circuitry of some LED fixtures and the switching mechanism of a mechanical timer.
Replace a mechanical timer with a digital programmable timer, which maintains accurate time even through brief power interruptions and allows multiple on/off cycles for a siesta schedule. If your LED fixture flickers when used with a timer, the LED controller may use PWM (pulse-width modulation) dimming that is incompatible with certain timer relay types — check the fixture manufacturer's recommendations for compatible timer types, or switch to a smart plug with a scheduling app (Kasa, TP-Link, or similar) for precise, phone-controlled scheduling.
5 My cyanobacteria (blue-green slime algae) keeps returning despite reducing light.
Possible cause: Cyanobacteria is not primarily a light-duration problem — it is primarily caused by low flow rates creating stagnant dead zones, excess organic waste in the substrate, and often a low nitrate-to-phosphate ratio (nitrate below 5 ppm while phosphate is elevated). Cyanobacteria can fix atmospheric nitrogen directly, giving it a competitive advantage in low-nitrate conditions.
Improve circulation by adding a small powerhead or repositioning existing flow to eliminate stagnant areas. Vacuum the substrate thoroughly to remove decomposing organic matter. Counterintuitively, dosing nitrogen (potassium nitrate) to bring nitrate to 10–20 ppm often suppresses cyanobacteria because higher nitrate levels allow competing plants and bacteria to outcompete it. A 3-day blackout followed by an erythromycin-based treatment (where legally available) is the fastest cure for severe outbreaks. Address the flow and nutrient balance after treatment to prevent recurrence.
6 My fish seem stressed and are hiding more since I changed the light schedule.
Possible cause: Fish are sensitive to sudden changes in their light cycle. A large abrupt shift — particularly a sudden increase in duration or intensity — can cause acute stress even if the new schedule is technically more correct than the old one.
Introduce photoperiod changes gradually. If you are increasing duration, add no more than 30 minutes per week. If your light fixture supports dimming, ramp intensity up at the beginning and down at the end of the photoperiod over 30–60 minute periods to simulate dawn and dusk — this is far less startling to fish than an abrupt switch from complete darkness to full intensity. Most modern smart LED fixtures include automated sunrise/sunset modes for exactly this purpose.
Glossary of Terms
- Photoperiod
- The total number of hours of light provided to an aquarium in a 24-hour period. The photoperiod is the primary controllable variable in algae management and plant growth. Most freshwater planted tanks require a photoperiod of 6–10 hours depending on CO2 availability and plant density. Fish-only tanks require 6–8 hours for biological rhythm regulation.
- PAR (Photosynthetically Active Radiation)
- A measurement of light intensity in the wavelengths usable by aquatic plants for photosynthesis (approximately 400–700 nm), expressed in micromoles per square metre per second (μmol/m²/s). Unlike lumens (which measure light as perceived by the human eye) or watts (which measure power consumption), PAR directly quantifies how much usable light plants receive. Low-light plants thrive at 15–30 PAR; high-light plants require 50–150+ PAR.
- Siesta Schedule
- A lighting strategy that splits the daily photoperiod into two blocks separated by a mid-day dark period — for example, 4 hours on, 4 hours off, 4 hours on. The siesta simultaneously allows early morning and evening viewing of the tank without exceeding daily light limits, and disrupts the photosynthetic recovery cycle of primitive algae while allowing higher-order plants to adapt and continue growing.
- Algae Blackout
- An emergency treatment for severe algae outbreaks — particularly green water — that involves covering the entire aquarium in opaque material and leaving it in complete darkness for 3 to 4 consecutive days. The absence of light kills free-floating algae and severely weakens attached algae without harming fish (which can fast safely for this period) or established plants (which can survive short dark periods on stored energy). The blackout must be followed by a reduced photoperiod to prevent recurrence.
- CO2 (Carbon Dioxide) Injection
- A technique used in high-tech planted aquariums to supplement the dissolved CO2 available to plants for photosynthesis. Because CO2 is the primary limiting factor for photosynthesis in most aquariums, injection allows plants to photosynthesise more efficiently and for longer durations, enabling longer photoperiods without triggering algae. CO2 is typically injected via a pressurised cylinder and regulator connected to a diffuser submerged in the tank.
- Circadian Rhythm
- The internal biological clock that regulates sleep, feeding, hormone production, and immune function in fish on a roughly 24-hour cycle. In aquariums, the light/dark cycle is the primary environmental cue that sets and maintains the circadian rhythm. Irregular or absent light schedules disrupt this rhythm, causing chronic stress, immune suppression, and behavioural abnormalities. Consistent photoperiods via a timer are the most effective way to maintain healthy circadian rhythm in aquarium fish.
- Cyanobacteria (Blue-Green Algae)
- A type of photosynthetic bacteria — not true algae — that forms blue-green, red, or purple slimy sheets on substrate, plants, and decorations. Often has a distinctive musty or earthy odour. Unlike true algae, cyanobacteria can fix atmospheric nitrogen directly from the water, making it particularly difficult to outcompete through nutrient management alone. Driven by poor flow, excess organic waste, and low nitrate-to-phosphate ratios. Treated with a combination of blackout, improved circulation, and sometimes erythromycin-based medication.
- Nutrient Imbalance
- A condition in which one or more dissolved nutrients — typically nitrogen (nitrate), phosphorus (phosphate), potassium, or iron — is present in a ratio that favours algae over plants. For example, very low nitrate combined with elevated phosphate creates conditions where cyanobacteria thrives. Excess phosphate with high light and no CO2 drives green spot algae. Balancing nutrients through fertilisation and water changes, combined with correct photoperiod management, is the foundation of algae control in planted tanks.
- Spectrum (Light Spectrum)
- The range of wavelengths emitted by an aquarium light, typically described in nanometres (nm). Different wavelengths drive different biological processes: blue light (450–470 nm) drives photosynthesis efficiently but also strongly promotes algae growth; red light (640–680 nm) is the most efficient wavelength for plant photosynthesis; green light (500–570 nm) is reflected by plants and least useful for photosynthesis. Full-spectrum white LEDs cover all wavelengths and are the practical standard for most freshwater planted tanks.
- UV Steriliser
- A filtration device that passes aquarium water past an ultraviolet light source (typically at 254 nm wavelength) to destroy the DNA of free-floating microorganisms — including green water algae cells, bacteria, and some parasites. Highly effective at permanently eliminating green water bloom when used correctly. Does not affect attached algae. Requires correct flow rate to allow adequate UV exposure time, and the UV bulb must be replaced annually as intensity degrades over time.
- Diatoms (Brown Algae)
- Single-celled organisms that form a brown, dusty coating on glass, substrate, and decorations — particularly common in new aquariums during the first 4 to 12 weeks of operation. Diatoms thrive in low-light conditions and high silicate levels (often present in new tap water or new substrate). They are largely self-limiting and typically disappear as the tank matures and silicates are consumed. Otocinclus catfish and nerite snails are effective biological control for persistent diatom growth.
- Low-Tech Planted Tank
- An aquarium plant setup that relies on ambient CO2 (dissolved naturally from the atmosphere and fish respiration) rather than injected CO2. Low-tech tanks have a lower maximum photosynthetic rate than CO2-injected tanks, which means they require shorter photoperiods (6–7 hours) and lower light intensity to remain in the balance where plants outcompete algae. A well-designed low-tech tank using appropriate plant species is highly stable and requires minimal intervention.
- High-Tech Planted Tank
- An aquarium plant setup that uses pressurised CO2 injection, high-intensity lighting, and regular fertilisation to drive rapid, lush plant growth. High-tech tanks can support longer photoperiods (8–10 hours) and more demanding plant species, but require careful management of the CO2, light, and nutrient balance. When any one of the three pillars falls out of balance, algae proliferates rapidly. Higher maintenance demands than low-tech setups, but capable of producing spectacular aquascape results.
Scientific References
- The Estimative Index (EI) Method — Tom Barr, Barr Report
- Photosynthetically Active Radiation (PAR) and Aquatic Plant Growth — Tropica Aquarium Plants
- Algae in the Aquarium: Causes and Treatments — Aquarium Science
- Circadian Rhythms in Fish — Reebs, S.G., Animal Behaviour
- Cyanobacteria in Freshwater Aquariums — Seriously Fish
- UV Sterilisers and Algae Control — Practical Fishkeeping