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Lighting is the engine that powers plant growth in aquariums. If you are new to planted tanks, it is easy to get overwhelmed by watts, lumens, and PAR charts. The good news is you do not need to be an engineer to grow healthy aquatic plants. You just need enough light intensity for your plant types, even coverage across the tank, the right color spectrum, and a simple daily schedule that avoids algae. This guide explains how many lights you actually need and exactly how long to leave them on, with clear examples for different tank sizes and goals.
Why light matters for aquarium plants
Plants use light to drive photosynthesis, which creates energy for growth and oxygen for your fish. Without enough light, plants stall, leaves yellow or melt, and algae can creep in. With too much light, algae can also explode because light is a growth accelerator for all photosynthetic life. That means success is not about blasting your tank with the brightest light you can buy. Success is about the right amount of light for your plants, delivered evenly and for the right number of hours each day.
How many lights do I need?
The number of light fixtures you need depends on the size and shape of your tank, the depth of the water, the kinds of plants you want to grow, and the strength of each fixture. Many tanks only need one high-quality LED bar. Larger or deeper tanks may do better with two lights to improve coverage and intensity. It is less about counting fixtures and more about achieving the correct intensity and spread at the plant level.
One bright fixture vs multiple fixtures
A single strong LED bar works well on small to medium tanks, especially if they are not very deep. On longer tanks, two bars can reduce shadows, light the front and back evenly, and give better penetration in taller aquariums. If your tank is 48 inches long or more, or deeper than 20 inches, two fixtures are often easier than pushing one light to its limits. Using two medium-strength fixtures can produce smoother coverage than one very intense spotlight.
Tank size, depth, and plant choice
Shallow tanks need less intensity than deep tanks because light fades as it passes through water. A 12 inch tall tank is forgiving. A 24 inch tall tank needs stronger lighting to reach the substrate where most plants sit. Your plant choice matters just as much. Low light plants like Anubias, Java fern, and many Cryptocoryne species will thrive with modest lighting. Medium light plants like stem plants, Vallisneria, and Scarlet temple need a bit more. High light carpets like Glossostigma, dwarf baby tears, and Monte Carlo need strong light and usually pressurized CO2. Choose plants to match your lighting comfort level, or choose lighting to match your dream plants.
Understanding intensity: PAR, lumens, and watts
PAR is the most useful measure because it measures the light plants can use, expressed in micromoles per square meter per second. At the substrate, low light plants are comfortable around 20 to 40 PAR, medium light at 40 to 70 PAR, and high light carpets at 70 to 100 PAR or more. Lumens measure brightness for human eyes, not plant use, but they can still be a rough guide when comparing lights of the same type. Watts are about power draw and should not be used to compare LEDs to fluorescent or across different brands.
If you do not have a PAR meter, you can still make good choices. Look for manufacturer PAR charts when available. As a very rough starting point for modern white LEDs, many hobbyists aim for about 20 to 30 lumens per liter for low light tanks, 30 to 45 lumens per liter for medium light, and 45 to 60 lumens per liter for high light, understanding that lumens are only a coarse guide. Always adjust based on plant response and algae levels.
Quick rules of thumb you can actually use
If your tank is shallow and you grow easy plants, one decent LED will usually do the job. If your tank is long or deep, or you want carpets and fast stems, two fixtures or one high output fixture is wiser. For most community planted tanks, aim to deliver roughly 30 to 50 PAR at the substrate. Start with the light dimmer at 50 to 70 percent if your fixture has one, then adjust in small steps over a few weeks while watching plant growth and algae.
Estimating fixtures by common tank sizes
On a nano tank of 5 to 10 gallons that is 10 to 12 inches tall, one small full spectrum LED is enough for low to medium plants. On a 20 gallon long that is 12 inches tall, one quality LED bar often covers the whole tank. On a 29 gallon or a 55 gallon that is 18 to 21 inches tall, many aquarists prefer two medium LED bars for even coverage, especially if growing stem plants. On a 75 gallon or taller tank, plan on either two strong bars or a high output fixture designed for deep tanks, especially if you plan carpet plants.
Spectrum and color temperature
Plants use many wavelengths, but they respond strongly to blue and red light. For planted tanks, full spectrum white LEDs with a color temperature between 5000K and 7000K produce natural-looking colors and healthy growth. The exact Kelvin number is not critical. Most aquarists prefer the crisp daylight look of 6000K to 6500K. A color rendering index of 80 or higher helps your fish and plants look realistic, not washed out or overly purple.
Do you need extra red and blue?
Many modern planted LED fixtures already include enough red and blue within their white channels or add a gentle mix of red diodes. You do not need overly colored grow lights to succeed. If your fixture allows channel control, you can slightly raise red for better reds in plants, but keep the overall balance natural. Good plant growth comes more from correct intensity and photoperiod than from color tweaking.
Photoperiod: how long to leave the lights on
Most planted tanks do best with 6 to 10 hours of light per day. A shorter period limits algae while plants establish. A longer period can be used once the tank is stable and plant mass is high. The safest plan is to start at 6 hours, observe for two weeks, and increase by 30 minutes per week if plants want more and algae is under control. Very long days are rarely helpful and often cause algae blooms, especially early on.
Starting schedule for a new tank
For the first month, aim for 6 hours per day at moderate intensity. This lets your filter and plants catch up with nutrients in the water. After four weeks, if plants look strong and algae is minimal, increase to 7 hours. Over another two to four weeks, walk it up towards 8 hours if needed. Many successful aquarists never go above 8 hours because plants will grow well during that window and algae pressure stays low.
With and without CO2
CO2 changes the equation. In a low tech tank without injected CO2, keep light moderate and the photoperiod around 7 to 8 hours, because plants are limited by available CO2 and cannot use very intense light efficiently. In a high tech tank with pressurized CO2, you can run higher intensity because CO2 is abundant. Even then, many high tech tanks are steady at 6 to 8 hours to keep algae in check. Always turn CO2 on about one hour before lights on and turn CO2 off about one hour before lights off.
Split photoperiods and the midday siesta
Some aquarists split the day into two blocks, such as four hours on in the morning, four hours off, then four hours on in the evening. This can work if it fits your viewing schedule, and it can help control heat and algae in some cases. Plants handle this just fine. If you try this, keep the total daily light similar to a continuous schedule and keep nutrients and CO2 stable across both light blocks.
Dimming and sunrise or sunset ramps
If your light supports dimming or gradual ramps, use them. A 30 to 60 minute ramp up and ramp down looks natural and reduces stress for fish. Ramps do not count fully as light time, so keep your main peak period within the 6 to 10 hour range, and use the ramps on both ends as gentle transitions.
Positioning and installation for best results
Place the light so it covers the entire length and width of your tank. In a very long tank, two shorter fixtures can be spaced to avoid dark corners. If your light is very strong, you can mount it slightly higher or dim it to reduce hot spots and improve spread. Make sure floating plants or wood are not blocking all the light to certain areas unless you are creating deliberate shade zones.
Tank lids, water clarity, and light losses
Glass lids, mesh tops, and mineral deposits can cut light by 10 to 30 percent. Clean the glass every week or two. Tannins, green water, and surface film also reduce penetration. Good water clarity, gentle surface movement, and clean covers help your plants receive the light you paid for. If your tank is very deep or covered, you might need to run the light a bit brighter to compensate.
Using timers, controllers, and syncing with CO2
The simplest way to get consistent results is to use a plug-in timer or a smart plug. Set the on and off times once and forget them. If you have CO2, set it to start one hour before the lights and end one hour before the lights go off. Consistency is more important than perfection. When lights, CO2, and feeding happen at the same time each day, your plants and fish settle into a healthy rhythm.
Fertilizer and light go together
More light means more plant growth and a higher need for nutrients. If you increase light intensity or extend the photoperiod, watch for signs of nutrient deficiency like pale new leaves or pin holes. Increase your fertilizer dose or frequency to match. In a low tech tank with low light, a weekly all-in-one fertilizer may be enough. In a high tech tank with strong light, daily or every other day dosing is often better. Light, CO2, and nutrients should rise together to keep balance.
Practical lighting examples by tank
Consider a low tech 20 gallon long tank that is 30 inches by 12 inches by 12 inches with Anubias, Java fern, and Crypts. One quality LED bar centered over the tank, set around 50 to 60 percent brightness, with 7 hours of light per day, will grow these plants well. Start at 6 hours for the first month, then increase if growth looks slow. Use a glass lid or keep the fixture close to the water for good penetration.
High tech 20 gallon with carpet
For a 20 gallon long with Monte Carlo carpet and pressurized CO2, use a strong LED bar or two slim bars for even spread. Set intensity to provide medium to high light at the substrate. Start at 6 hours per day with a 30 minute ramp up and down. Turn on CO2 one hour before lights on. Keep nutrients steady with regular dosing. Once the carpet roots and spreads, you can try extending to 7 or 7.5 hours if algae is under control.
55 gallon community with stems
A standard 55 gallon is 48 inches long and about 21 inches tall. Two medium strength LED bars across the top often produce better coverage than one bright bar, especially front to back. Position them so the beams overlap slightly. Aim for moderate intensity to deliver around medium PAR to the substrate. Run 7 to 8 hours per day. If taller plants cast heavy shade, consider raising the lights a little or trimming to keep lower plants happy.
Nano cube with a carpet and shrimp
A 10 to 15 inch cube is deeper than you might think because it is as tall as it is wide. Choose a compact light designed for depth and set it to moderate intensity to avoid algae in a small volume. If growing a carpet, add CO2 and keep the photoperiod at 6 to 7 hours. Shrimp appreciate stable conditions and gentle sunrise and sunset ramps. For a low tech shrimp and moss tank, reduce intensity and use 7 to 8 hours daily.
Acclimating plants to light
When you upgrade lights or add new plants, do not jump straight to maximum power. Start at 40 to 60 percent intensity and 6 hours per day for two weeks. Watch for signs of stress such as algae dust on glass, stringy green growth, or plants bleaching. Increase intensity in small steps of 5 to 10 percent or extend the photoperiod by 30 minutes per week. Slow, steady changes keep your tank stable.
Troubleshooting and algae control
Too much light shows up as fast algae growth, glass needs cleaning every few days, plants may turn pale if nutrients lag, or leaves may develop spot algae. Too little light shows as leggy stems stretching upwards, slow or no new leaves, and carpets lifting or melting. Adjust only one thing at a time. If you increase light, consider increasing CO2 or nutrients. If you reduce light, give plants two weeks to respond before deciding the next step.
If algae appears, what should you change?
First, shorten the photoperiod by one hour and clean the tank. Remove visible algae, clean the glass, and perform a large water change. Second, slightly dim the light or raise the fixture if possible. Third, check nutrients and CO2 stability. Many algae problems are caused by very bright light with unstable CO2 or low nutrients. Finally, increase flow and keep filters clean so plants receive a steady supply of CO2 and nutrients. After a week or two, reassess and make small adjustments rather than big swings.
Measuring light without fancy tools
If you do not have a PAR meter, you can still estimate. Many phones can read lux with a light meter app. While lux is not the same as PAR, you can use it as a reference for your own tank. If you can safely measure lux at the substrate through a clear bag to keep the device dry, record the number and compare after changes. As a rough idea for white LEDs, dividing lux by about 54 can estimate PAR, though this varies by spectrum. The absolute number matters less than consistency. If your reading doubles, your plants feel a big change too.
Common questions
Floating plants can reduce algae because they compete for nutrients and shade the tank. They also soften harsh light and protect fish. If you keep floaters, remember that fast growth can quickly block light from below, so thin them regularly. Floaters also allow you to run slightly brighter fixtures without blasting low plants. Many people use them as a gentle buffer against algae while the tank matures.
Do you need light at night?
No, do not leave the plant light on at night. Plants and fish need a dark period to rest. Night lighting can fuel algae and stress fish. If you want to view the tank after lights out, use a dim room light briefly or a very weak blue moonlight only for a short time. Keep the main plant light off during the night.
When should you upgrade your light?
Upgrade if your plants are healthy but cannot thrive even after you increase the photoperiod to about 8 hours, raise intensity sensibly, and keep nutrients and CO2 steady. If carpets will not start, tall stems are always leggy, or deep tanks have dark bottoms, a stronger or second fixture can help. Always try better positioning, cleaning covers, and improving water clarity before buying new lights, because these simple fixes often recover a lot of lost intensity.
Putting it all together: a simple formula for success
Choose a light that fits your tank length and depth. Aim for one LED bar on small to medium shallow tanks and consider two bars for long or deep tanks. Pick a spectrum around 6000K and do not obsess about color channels. Start with 6 hours per day at moderate intensity. Increase slowly only if plants ask for more and algae remains low. Match your plant selection to your light level so you do not fight nature. Keep glass, lids, and water clean to deliver the light you already have. Use a timer so every day is the same. Balance light with CO2 and nutrients so plants, not algae, win the race.
Conclusion
You do not need a complex setup to grow beautiful aquarium plants. Most tanks succeed with one well-chosen LED light, a sensible 6 to 8 hour schedule, and clean, even coverage. Larger or deeper tanks may benefit from two fixtures for better spread. Let your plant goals guide how much intensity you need. Keep the spectrum in the daylight range, ramp up gently, and make small changes while watching how plants and algae respond. If you keep the light consistent and balanced with CO2 and nutrients, your plants will reward you with steady growth, clear water, and a tank that looks alive and healthy every day.