Aquarium Light Intensity and Duration for Planted Tanks

Intensity and photoperiod are two different dials

Intensity is how much light reaches the plants at any moment. It depends on the fixture's output, the dimming setting, the height of the light, and how deep the water is.

Photoperiod is the number of hours the light is on each day.

Plants respond to the combination. A bright light for a short period and a softer light for a longer period can deliver similar total light, but they do not behave the same. Very high intensity can outpace the CO2 and nutrients available, which tends to favor algae. A very long photoperiod gives algae more hours to grow, often with little extra benefit to plants.

If your light has a dimmer, think of intensity as the main growth dial and photoperiod as the stability dial. Set a sensible, consistent photoperiod first, then adjust intensity in small steps.

How long should the lights be on in a new planted tank?

New tanks are where light problems usually start. Plants are adjusting to submerged conditions, the filter is still maturing, and algae spores are ready to take advantage of any imbalance.

Begin with a shorter photoperiod. Tropica's startup guidance is 6 hours a day for the first two to three weeks, then 8 to 10 hours. Many hobbyists use a similar range of roughly 6 to 8 hours to start. These are rules of thumb, not fixed requirements, and your tank's response matters more than the exact figure.

For intensity, start at a moderate dimmer setting rather than full power, especially in a tank without injected CO2. Our lighting selection guide explains why the same fixture behaves differently in shallow and deep tanks.

Ramping up gradually

Once plants show new growth, such as fresh leaves, new runners, or roots, you can begin to increase. A calm approach:

1. Change one thing at a time. Raise either the photoperiod or the intensity, not both.
2. Use small steps. Add around 30 minutes of photoperiod, or a modest step on the dimmer.
3. Wait and watch. Give each change a week or two before judging it. Plants and algae both take time to respond.
4. Step back if algae increases. Return to the last setting that was stable.

As a rule of thumb, many established tanks run somewhere around 7 to 10 hours a day. Longer photoperiods are rarely necessary and tend to raise the risk of algae.

Does a planted tank need a lighting siesta?

Some keepers split the photoperiod with a "siesta": lights on in the morning, off for a few hours midday, then on again in the evening. Supporters feel it helps with algae or lets them view the tank at two times of day. It is an option, not a requirement, and many healthy planted tanks run a single continuous block. If you try a siesta, keep the total hours similar to what worked before, and make sure any CO2 schedule matches both lit periods.

Balancing light with CO2 and nutrients

Light drives how fast plants try to grow. CO2 and nutrients determine whether they can actually do it.

• More light raises demand. A brighter tank needs more carbon and more fertilizer to keep plants healthy.
• In low-tech tanks, the carbon supply is limited, so moderate light is usually the stable choice.
• In CO2-injected tanks, CO2 should be at working levels when the light reaches full intensity. Many keepers start injection an hour or more before the lights come on.

When something is short, plants slow down and algae often fills the gap. That is why adding more light to a struggling tank frequently makes things worse. Read our CO2 system guide and fertilizer basics to balance the other two sides.

What are the signs of too much light?

• Algae appears quickly on glass, hardscape, and older leaves, especially soon after you raised intensity or hours.
• Green spot or hair algae shows up on slow-growing plants such as Anubias.
• Plants near the surface look pale or bleached while lower plants seem fine.
• Algae returns soon after each cleaning.

What are the signs of too little light?

• Stem plants grow tall and thin with wide gaps between leaves.
• Lower leaves yellow and drop as plants stretch toward the light.
• Carpeting plants lift and grow upward instead of spreading.
• Red plants stay green or brown.
• Growth stalls even though nutrients and CO2 are adequate.

Be careful with diagnosis. Some of these signs overlap with nutrient deficiencies or low CO2, so change one variable at a time.

How light links to algae

Algae is opportunistic. It can use light, nutrients, and carbon faster than slow-growing plants when conditions swing. Hair algae, for example, is often associated with excess lighting, and many algae issues are linked to imbalances rather than a single cause. Reducing photoperiod or intensity is one of the gentlest first steps when algae appears, because it slows algae without adding anything to the water. See the algae troubleshooting guide for a type-by-type approach.

Common mistakes

• Running a new tank at full intensity for 10 or more hours from day one.
• Changing intensity, duration, CO2, and fertilizer all at once, then not knowing which change helped.
• Adding more light to fix slow growth when CO2 or nutrients are the limiting factor.
• Leaving lights on irregularly, such as manual switching that varies from day to day.
• Treating published photoperiod numbers as rules rather than starting points.

Next steps

Put the schedule on a timer so it stays consistent, using our timers and lighting schedules guide. If you are still choosing a fixture, start with the lighting selection guide, browse the lighting hub, or see our light picks for 10-gallon, 20-gallon, and 40-gallon tanks. For a timer that can also switch a CO2 solenoid, see the timers hub or our aquarium light timer picks, which list each timer's rated load.