A standard aquarium light should run 8 to 10 hours daily, with the exact duration depending on plant load, tank age, and ambient room light.
For the full breakdown, see our best 72 Inch Aquarium Light guide.
The right photoperiod keeps your fish healthy, your plants growing, and your glass clean. Here is the breakdown by tank type, how to set it up with a simple timer, and what to do when you spot the first green film on the glass—because the fix is often just cutting an hour or two.
Standard Light Durations By Tank Type
Most freshwater tanks land in the 8-to-10-hour window. Reef tanks push higher because corals demand more light, while new setups need a deliberately short schedule to starve off the algae bloom that follows equipment setup.
- Fish-only tanks: 8 hours. No live plants to feed, so longer light only grows algae.
- Low-light planted tanks: 7 to 9 hours. Lower intensity means slower growth and less risk.
- High-light planted tanks with CO₂: 8 to 10 hours. The injected CO₂ helps plants outcompete algae at the higher end of the range.
- Reef tanks: 8 to 12 hours. Coral photosynthesis demands more light, but the duration depends on coral species and light intensity.
- New or algae-prone tanks: Start at 4 to 6 hours. Increase in 30-minute increments every two weeks if no algae appears.
Tanks placed near windows or in bright rooms need 4 to 6 hours of artificial light even in established setups—the ambient daylight already contributes to the total photoperiod.
Setting the Timer – The Only Reliable Way
A mechanical or digital outlet timer is the single best investment for consistent photoperiods. Fish and plants run on a biological clock; erratic light stresses them and creates algae windows.
For a new tank, start the timer at 6 hours and leave it untouched for two full weeks. If no algae appears, bump it to 7 hours and wait another two weeks. Repeat until you reach the target duration. If algae shows up at any point, drop back one hour and hold there—that is your tank’s ceiling.
Intensity matters as much as duration. Start LED fixtures at 20 to 40 percent brightness (50 to 70 percent for high-output fixtures) and increase in 10 percent steps only if you see plants stalling. Raising the fixture height reduces intensity naturally and is often the cleaner fix when algae appears near the top of the tank.
Common Mistakes That Ruin The Schedule
These errors are why most lighting problems are self-inflicted and fixable in five minutes.
- Running lights 24/7. Plants need darkness to respire. No dark period means no oxygen production and rampant algae.
- More than 12 hours. Fish need a rest cycle. Over 12 hours of light stresses them and triggers almost every algae type.
- Ignoring ambient room light. A 10-hour light fixture in a room that already gets 8 hours of window light means an 18-hour total photoperiod. Cut the fixture back proportionally.
- Making sudden changes. Dropping from 10 hours to 6 hours overnight shocks the biological balance. Adjust by 30 minutes per week.
When Algae Appears – The Correction Sequence
When you see the first green haze on the glass or string algae on plants, do these in order:
- Reduce the timer by 1 hour immediately.
- Wait one week. If the algae stops spreading but doesn’t disappear, hold the duration.
- If algae continues, reduce another hour. Continue until the tank stays clean for two weeks.
- Once clean, increase duration by 30 minutes per week only if plants show signs of slowing growth (leaning, yellowing lower leaves).
- If the tank stays clean at the shorter duration, that is your correct photoperiod. Do not push for a longer schedule just because the charts say “8 to 10 hours.” Every tank has a different sweet spot.
Algae that appears despite a 4-hour photoperiod points to a different cause: excess nutrients (overfeeding, overstocking, low filtration) or ambient light. Rule those out before assuming the fixture is the problem.
Lighting Hardware Tips
LED fixtures dominate modern tanks because they run cool, last years, and let you dial spectrum and intensity independently. The light must include wavelengths between 400 and 700 nanometers—the photosynthetic range—which most aquarium-specific LEDs already cover. Keep the light at least a few inches above the water surface if it is not an LED (older fluorescent and metal-halide fixtures generate heat that can cook the water column). Clean the fixture and glass cover monthly; salt creep and dust block 10 to 20 percent of light output, and that lost intensity is why some tanks look dimmer a year in.
References & Sources
- Aqueon. “Healthy Aquarium Lighting Guide.” Covers duration recommendations by tank type and intensity adjustment.
- Aqueon. “Why Is a Light Schedule Important for My Aquarium?” Explains biological clock and timer setup logic.
- The Spruce Pets. “How Long Should Aquarium Lights Be Left On?” Detailed photoperiod ranges for freshwater and reef tanks.