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Getting the aquarium light schedule right keeps fish calm, plants healthy, and algae under control. Too much light fuels algae and stresses animals. Too little light weakens plants and dims coral color. You do not need an expensive controller to succeed. You need a consistent daily plan that fits your tank type and your routine. This guide gives clear hour ranges, explains why they work, and shows you how to fine tune your schedule without guesswork.
Light Duration Basics You Must Know
Photoperiod sets the daily rhythm
Photoperiod is the number of hours your main aquarium light is on each day. Fish and invertebrates follow daily cycles. Plants and corals switch between photosynthesis in light and respiration in darkness. A steady light and dark cycle reduces stress, supports feeding and rest, and keeps growth steady. The key is regular on and off times. Random changes confuse animals and encourage algae.
Duration, intensity, and spectrum work together
Light duration is only one part of the picture. Intensity describes how strong the light is. Spectrum describes color distribution. You can run a longer day with moderate intensity or a shorter day with high intensity. If you increase intensity, you often reduce duration to keep the total light dose reasonable. For beginners, aim for a moderate intensity that matches your livestock, then set a stable duration. Avoid trying to compensate for poor intensity with very long days. That often triggers algae.
Quick Answer by Tank Type
Freshwater fish only tanks
Fish do not need bright light for health. Light is for your viewing and to support live plants if you have them. For fish only tanks without plants, run 6 to 8 hours of main lighting daily. If you enjoy a longer viewing window, keep intensity low to moderate and add a dim room lamp before or after the main cycle rather than extending the tank light. Maintain at least 12 hours of darkness or low ambient light for rest.
Freshwater planted tanks
Low tech planted tanks without injected CO2 do best at 6 to 7 hours daily. This keeps algae pressure low while letting slow growers thrive. If growth is steady and algae is minimal for a full month, you can try 7.5 hours.
Medium tech planted tanks with moderate light and either liquid carbon or very stable nutrient dosing do well at 7 to 8 hours. Extend only if plant growth is strong and algae is under control.
High tech planted tanks with injected CO2 and consistent fertilization often target 7 to 8 hours total. Some advanced setups run up to 9 hours, but only when CO2 is rock solid and nutrients are balanced. If any algae appears, cut back by 60 to 90 minutes before changing anything else. Healthy plants will adapt faster than algae.
Saltwater fish only and reef tanks
Fish only with live rock tanks can run 6 to 9 hours. The light is for viewing and coraline growth. Avoid very long days to limit nuisance algae.
Reef tanks need a longer window to support photosynthetic corals. Run an overall day of 8 to 12 hours with a peak period at higher intensity. A common plan is 2 hours ramp up, 4 to 6 hours at peak, and 2 hours ramp down.
Soft corals often do well with 8 to 10 hours total. LPS corals usually like 8 to 10 hours with a 4 to 6 hour peak. SPS corals benefit from 9 to 12 hours total, with 5 to 6 hours at peak intensity. Many reef keepers run blue channels longer than white channels to extend viewing and color without blasting corals all day. Example: Blue 10 hours with a 5 hour peak; White 6 to 8 hours overlapping the middle of the day.
Refugiums and algae scrubbers
For macroalgae in a refugium, run 12 to 16 hours opposite the display lighting. This stabilizes pH and reduces nutrient swings. Some systems run the refugium 24 hours, but most see good results with 12 to 16 hours. If macroalgae pale or stop growing, shorten the day or increase nutrients and iron as needed.
Build a Daily Schedule That Works
Pick on and off times that fit your life
Choose times that match when you are home. A consistent schedule is more valuable than a perfect theoretical plan. If you are home in the evening, set lights to start later in the day. For example, 2 pm to 9 pm for a planted tank, or 1 pm to 10 pm for a reef with blue channels extended. Avoid waking fish with lights at dawn if you are never around to enjoy it.
Use ramps to reduce stress
Sudden on and off can startle fish and corals. If your light supports ramping, program a 30 to 120 minute sunrise and sunset. If not, use a dim room light for 15 minutes before and after the tank light to soften transitions. This eases the change and lowers stress during feeding and maintenance.
Try a split schedule only when needed
Some planted tanks benefit from a midday siesta. You run 3 to 4 hours in the morning, a 2 to 4 hour pause, then 3 to 4 hours in the evening. This can cut algae during early phases or in hot rooms. Only use a split if a single block causes algae. Many tanks run best on one continuous block.
Darkness matters
Animals need a solid dark period to rest. Aim for 10 to 12 hours of darkness. Avoid bright night lights. If you enjoy a brief moonlight period, keep it very dim and short, 1 to 2 hours, then switch off. Continuous blue light through the night can disrupt rest and may encourage algae on glass.
Read the Signs and Fine Tune
Signs of too much light
Rapid green film on glass, hair algae on leaves or rocks, a dusting of cyanobacteria on sand, fish hiding or darting at lights on, and washed out plant or coral color are classic signals. On planted tanks, if you see algae with stable nutrients and CO2, cut the photoperiod by 60 to 90 minutes and increase water changes for two weeks. On reefs, reduce peak time by 1 to 2 hours or lower whites before touching the total day length of the blue channels.
Signs of too little light
Plants grow leggy, leaves turn pale, carpeting plants lift, and red plants fade to green. Corals lose vibrancy or extend polyps without growth, then recession follows. On fish only tanks, weak light mainly affects your viewing experience. The fix is gradual. Increase duration by 30 minutes per week or increase intensity by 10 to 15 percent and watch for two weeks.
Control algae without starving your tank
Do not solve algae by slashing light to very short days while leaving nutrient problems in place. Balance wins. Maintain steady fertilization for plants and proper nutrient export for reefs. Adjust light in small steps. Siphon algae, clean filters, and add more frequent water changes until the system stabilizes. Consistency is the fastest path to balance.
New Tanks and New Lights
Break in period for new tanks
New tanks often go through diatoms and soft algae blooms. Keep the photoperiod short at first. For planted tanks, start at 6 hours for the first 2 to 3 weeks. For reefs, start at 7 to 8 hours total with a short peak. Increase only after the tank shows stable parameters and minimal algae. Patience here saves months of cleanup later.
Photoacclimation for livestock or fixture changes
When you add a stronger light, introduce sensitive plants, or add corals, reduce the photoperiod or intensity first. One simple method is to reduce intensity to 50 to 70 percent and shorten the day by 1 to 2 hours. Then raise intensity by 5 to 10 percent each week and add back 15 to 30 minutes per week until you reach your target. Watch color and polyp extension on corals and new growth tips on plants to confirm progress.
Room Light, Windows, and Maintenance
Natural sunlight and windows
Direct sunlight can fuel algae and heat swings. Keep aquariums away from south facing windows or use blinds. If the tank gets some ambient daylight, do not count that as your main photoperiod. Keep your scheduled light consistent. If sunlight is unavoidable, shorten your main light by 30 to 60 minutes and add more frequent glass cleaning until algae stabilizes.
Do room lights count
Normal room lights are usually too weak to drive plant or coral growth, but they do affect fish sleep. Avoid very bright room lighting during the night. If you need to enter the room, use a small lamp and keep it brief. Your fish will rest better and show better color during the day.
Lighting during maintenance
Turn off lights during large water changes. Sudden exposure of corals or plant leaves can stress tissue. Lights off also calm fish and reduce jumping during maintenance. Turn lights back on after refilling and dechlorinating. If you deep clean, consider a shorter light day to reduce disturbance.
Plants, Corals, Nutrients, and CO2
Balance with CO2 and fertilization
Plants and corals use light to drive growth, but they also need carbon and nutrients. In planted tanks, adequate CO2 is the difference between a clean 7.5 hour day and a messy 10 hour day. If CO2 is low, extra hours only grow algae. In reefs, stable nitrate, phosphate, and trace elements support color and calcification. If nutrients are near zero, long intense days bleach corals. Match your photoperiod to your nutrient and CO2 strategy, not the other way around.
Focus your peak on the productive window
Photosynthesis saturates after a point. Most systems benefit from a moderate peak of 4 to 6 hours with ramps on both sides. This gives plants and corals a strong productive window without overshooting. If you want a longer viewing period, extend lower intensity blue or warm channels before and after the peak.
Measuring Light and Making Adjustments
Use PAR, LUX, or simple tests
In reefs and high light planted tanks, a PAR meter is ideal. If that is not available, a basic LUX meter or a smartphone app can help track relative changes. Take readings at known spots and compare during adjustments. Even without tools, you can observe shadows, plant pearling near the end of the day, and coral polyp extension patterns to gauge whether light is adequate.
A two week adjustment method
Change only one variable at a time. Adjust the photoperiod by 30 to 60 minutes or intensity by 10 percent. Hold for two weeks. During that time, clean glass every few days, record any algae growth, and note animal behavior. If algae grows faster, reverse half the change. If growth improves and algae slows, keep the change. This method is slow but reliable.
Practical Schedules You Can Copy
Freshwater fish only
Run 7 hours at moderate intensity, for example 2 pm to 9 pm. If algae grows, drop to 6 hours. If you want more viewing time, turn on a floor lamp for 30 minutes before and after the tank lights.
Low tech planted tank
Start at 6.5 hours, such as 3 pm to 9:30 pm. Keep nutrients steady and avoid big intensity spikes. After one month of stable growth, try 7 hours. If hair algae appears, go back to 6.5 hours for two weeks and increase water changes.
High tech planted tank with CO2
Run 7.5 hours total with a 60 minute ramp up and 60 minute ramp down. Keep a 5 hour peak in the middle. Example: 2 pm ramp up, 3 pm to 8 pm peak, 8 pm to 9:30 pm ramp down. Ensure CO2 is stable from the start of the ramp and well into the peak. If any algae appears, reduce the peak by 30 minutes first.
Reef tank mixed corals
Run blue channels for 10 hours with a 2 hour ramp up and down. Run white channels for 6 to 8 hours centered on the middle of the day. Example: Blues 12 pm to 10 pm, whites 2 pm to 8 pm. If SPS pale, reduce white intensity or shorten the white window by 1 hour. If soft corals stretch, increase blue intensity slightly or extend the blue window by 30 minutes.
Refugium
Run 14 hours opposite the display. If the display is on from 1 pm to 10 pm, run the refugium from 10 pm to 12 pm. If macroalgae stalls, check iron and nitrate, then adjust duration by plus or minus 2 hours.
Special Cases
Betta and low light fish
Keep light gentle and limit to 6 to 7 hours. Floating plants or a dim area help. Provide at least 12 hours of darkness for rest.
Goldfish tanks
Goldfish stir up waste which fuels algae. Use 6 to 7 hours and strong filtration. If algae persists, keep the same duration and correct nutrients through water changes instead of cutting light more.
Shrimp tanks
Shrimp enjoy stable, clean surfaces. Start at 6 hours, add moss and biofilm supports, and extend to 7 hours only if algae is minimal and plants need more time.
Small SPS nano reefs
Nano tanks swing fast. Keep a 9 to 10 hour day with a 4 to 5 hour peak. Prioritize stability over long viewing windows. Frequent small water changes help.
Common Mistakes to Avoid
Chasing algae with drastic cuts
Slashing light from 9 hours to 4 hours may stop algae briefly but will harm plants and corals. Make small, patient changes and fix root causes like excess nutrients or weak CO2.
Leaving lights on for days during algae battles
Blackouts can help specific blooms, but use with care. For planted tanks, a 2 to 3 day blackout can reset stubborn algae, followed by a shorter 6 to 7 hour schedule and improved maintenance. For reefs, avoid full blackouts unless targeting dinoflagellates and follow a proven protocol. Return to normal gradually.
Ignoring darkness
Animals need rest. Night lighting should be brief and dim. Continuous night light raises stress and can disrupt breeding and growth.
Step by Step Setup Plan
Set your baseline
Pick the recommended hour range for your tank type. Program a timer. Add a 30 to 60 minute ramp if available or a short room light buffer before and after the main light.
Observe for two weeks
Clean glass at the same interval, watch animal behavior, and note any algae. Keep feeding and maintenance steady. This creates a fair test of the photoperiod.
Adjust in small steps
If growth is weak, add 30 minutes. If algae increases, subtract 30 to 60 minutes. Change only one variable at a time. Hold each change for two weeks before making another.
Lock the routine
Once you find a schedule that works, keep it consistent. Stability is the foundation of a healthy aquarium.
Simple Answers to Frequent Questions
How many hours should aquarium lights be on
Most tanks do well between 6 and 10 hours. Fish only sits at the low end. Planted and reef tanks need the higher end with a defined peak period.
Can I run lights 12 hours for plants
For most freshwater plants, 12 hours is too long unless intensity is low and growth is balanced. Try 7 to 8 hours first. Extend only if algae is minimal and plants ask for more.
Is longer blue light safe for reefs
Yes, many reef tanks run blues longer. Keep the higher intensity peak shorter and do not leave blues on overnight. Watch coral color and polyp extension to confirm comfort.
Do I count ambient daylight
No, unless the tank receives strong direct sun. Keep your programmed light consistent and control sunlight with blinds if needed.
What about seasonal changes
You can simulate longer summer days and shorter winter days, but it is optional. Most aquariums run best on the same schedule year round.
Conclusion
There is no single perfect number of hours for every aquarium, but there is a clear method to find yours. Start with proven ranges for your tank type. Keep on and off times consistent. Use ramps to keep animals calm. Watch for signs of excess or shortage and adjust by small steps. Match your photoperiod to your intensity, nutrients, and CO2. With a steady routine and patient tuning, you will reach a clean, stable balance where fish rest well, plants or corals grow steadily, and algae no longer drives your decisions.