Planted tank lighting: photoperiod and spectrum

The lighting is the single most important variable in the planted freshwater tank, and the lighting is also the single most common source of the planted tank's problems. The planted tank with the right lighting is a planted tank that is thriving, that has healthy plant growth, that has minimal algae, and that has a stable, beautiful ecosystem. The planted tank with the wrong lighting is a planted tank that is struggling — the plants are stunted or melted, the algae is taking over, the CO2 is wasted, and the visual result is the brown-water, hair-algae disaster that is the dreaded "newbie tank."

This article is going to walk you through the practical lighting for the planted freshwater tank — the photoperiod (the hours of light per day), the spectrum (the wavelengths of light), the intensity (the amount of light reaching the plants), the equipment options (the modern LED fixtures, the T5/T8 fluorescents, the metal halide), the plant-specific considerations (the low-light plants, the medium-light plants, the high-light plants, the carpeting plants), and the four mistakes that turn a planted tank into an algae farm. I am a pond and aquarium keeper with 20+ years of experience ranging from simple goldfish tanks to high-tech planted tanks, and the lighting is the variable I get asked about most often. The cases that go well share a common feature: the owner has matched the lighting to the plant selection, has set the photoperiod appropriately, and has adjusted the lighting based on the plant response and the algae response. The cases that go poorly share a different common feature: the owner has installed the brightest light available, has run the light 10–12 hours per day, has ignored the plant-specific requirements, and has produced the algae disaster that is the predictable result.

I want to be clear at the start: more light is not better. More light is, in many cases, worse. The algae blooms in proportion to the light, and the algae blooms faster than the plants. The light that is too intense is the light that produces the algae, and the algae is what the owner has to manage. The right amount of light is the amount that supports the plants without producing the algae, and the right amount is often less than the owner thinks.

The three variables: photoperiod, spectrum, and intensity

The lighting for the planted tank is built on three variables, and the three variables must be considered together when choosing the lighting and when troubleshooting the planted tank.

The photoperiod (the hours of light per day). The photoperiod is the easiest of the three to control, and the photoperiod is the variable that the owner can adjust without buying new equipment. The photoperiod for the planted tank:

• Low-light planted tanks (Anubias, Java fern, Java moss, Cryptocoryne): 6–8 hours per day.

• Medium-light planted tanks (most stem plants, the easy carpeting plants like Marsilea): 7–9 hours per day.

• High-light planted tanks (the carpeting plants like Hemianthus callitrichoides, the demanding stem plants like Rotala wallichii, the red plants): 8–10 hours per day.

The photoperiod of more than 10 hours per day is the photoperiod that is producing the algae, and the photoperiod should be reduced to the 6–8 hour range if the algae is blooming. The photoperiod of less than 6 hours per day is the photoperiod that is not supporting the plants, and the photoperiod should be increased (along with the intensity) if the plants are not growing.

The spectrum (the wavelengths of light). The spectrum is the color composition of the light, and the spectrum affects the plant growth and the visual appearance of the tank. The spectrum is measured in Kelvin (K), with the lower Kelvin (3000K) being warm/yellow, the middle Kelvin (6500K) being daylight white, and the higher Kelvin (10000K+) being cool/blue. The spectrum for the planted tank:

• The full spectrum (6500–10000K): the most natural-looking spectrum for the planted tank, and the spectrum that supports the plant growth well. The 6500K to 7500K range is the most common choice for the freshwater planted tank.

• The warm spectrum (3000K–4000K): a more yellow light that is used in some aquascapes (the Takashi Amano-style tanks often use a 5000–6500K range) to enhance the red and the orange colors of the fish and the plants.

• The red/blue spectrum (with red and blue LEDs): the most efficient for plant growth (the red and the blue are the wavelengths the plants use most for photosynthesis), but the visual appearance is not as natural-looking as the full spectrum.

The right spectrum for the planted tank is the full spectrum (6500–7500K), with the option to add some red or blue for the plant growth and the visual enhancement. The single-color or narrow-spectrum lights (the all-blue, the all-red) are not appropriate for the planted tank.

The intensity (the amount of light reaching the plants). The intensity is the most important variable, and the intensity is the variable that is the most commonly misjudged. The intensity is measured in PAR (photosynthetically active radiation) at the substrate level, and the intensity is the variable that determines whether the plants are receiving the right amount of light for the species.

The intensity for the planted tank, measured at the substrate:

• Low light: 15–30 PAR at the substrate. Suitable for the low-light plants (Anubias, Java fern, Cryptocoryne).

• Medium light: 30–50 PAR at the substrate. Suitable for the medium-light plants (most stem plants, the easy carpeting plants).

• High light: 50–100+ PAR at the substrate. Suitable for the high-light plants (the demanding carpeting plants, the red plants, the demanding stem plants).

The intensity is determined by the wattage of the light (for the older fluorescent and metal halide fixtures), by the PAR output of the LED (for the modern LED fixtures), and by the distance from the light to the substrate (the closer the light, the higher the intensity at the substrate; the farther the light, the lower the intensity). The owner of a new planted tank should not be guessing — the owner should be measuring, with a PAR meter, or should be researching the specific fixture's PAR output at the relevant distance.

The light that is too intense is the light that is producing the algae, and the light that is too weak is the light that is not supporting the plants. The right light is the light that supports the plants without producing the algae, and the right light is specific to the plant selection.

The equipment options

The modern options for the planted tank lighting:

The modern LED fixture (the current standard). The LED fixture is the most popular choice for the planted tank, and the LED fixture is the right choice for most planted tanks. The LED fixture is energy-efficient (lower electricity cost), is long-lasting (50,000+ hours), is dimmable (the intensity can be adjusted), and is available in a range of spectrums (from the all-white to the multi-channel RGB). The leading brands are:

• Chihiros (A2, WRGB, RGB Vivid). The most popular planted tank LED brand. The Chihiros fixtures are known for their high PAR output, their full spectrum, and their dimmable controllers.

• Twinstar (S-LINE, III). A high-end LED brand, popular for the high-tech planted tanks.

• ONF (Flat Nano, Flat One). A premium LED brand, popular for the nano and the small planted tanks.

• Finnex (Fury, Planted+). A budget-friendly LED brand, popular for the entry-level planted tank.

The right LED fixture for the planted tank depends on the tank size, the plant selection, and the budget. The general rule: the LED fixture should provide 30–50 PAR at the substrate for the medium-light planted tank, and the fixture should be dimmable so that the intensity can be adjusted.

The T5 fluorescent (the older standard, still in use). The T5 fluorescent fixture was the standard for the planted tank for many years, and the T5 fluorescent is still in use in many established planted tanks. The T5 fluorescent provides a good spectrum, a reliable output, and a moderate cost. The disadvantages are the higher electricity cost (compared to LED), the shorter useful life (12–18 months, with the output dropping over time), and the larger fixture size.

The T8 fluorescent (the even older standard, mostly obsolete). The T8 fluorescent is the older fluorescent standard, and the T8 fluorescent is being phased out in favor of the T5 and the LED. The T8 fluorescent is appropriate for the very low-light planted tanks (Anubias, Java fern), but the T8 is not appropriate for the medium- or high-light planted tanks.

The metal halide (the high-end option for the demanding tanks). The metal halide is the high-intensity discharge (HID) light that produces a very high PAR output, and the metal halide is the right choice for the very high-light planted tanks (the Dutch-style tanks with the demanding stem plants, the large tanks with the high-light carpeting plants). The metal halide produces a significant amount of heat (the tank may need a chiller), the metal halide uses a lot of electricity, and the metal halide is being replaced by the high-end LED fixtures in most planted tanks. The metal halide is still used in the very large tanks (300+ gallons) where the LED fixtures do not provide enough coverage.

The plant-specific considerations

The right lighting depends on the plant selection, and the right lighting for the high-tech carpeting plants is different from the right lighting for the low-light Anubias.

The low-light plants. Anubias, Java fern, Java moss, Cryptocoryne, Bolbitis. These plants grow in the shaded areas of the natural environment, and the plants do not need intense light. The low-light plants do well with 15–30 PAR at the substrate, and the low-light plants do not benefit from additional light (the additional light is wasted on the plants and is producing the algae). The right light for the low-light planted tank is a low-intensity light (a single 24-watt LED fixture over a 10-gallon tank, for example), with a photoperiod of 6–8 hours per day.

The medium-light plants. Most stem plants (Hygrophila, Rotala, Ludwigia, Bacopa, Cabomba), the easy carpeting plants (Marsilea, Glossostigma), the mid-ground plants (Ludwigia repens, Alternanthera reineckii), and the background plants (Vallisneria, Sagittaria). The medium-light plants do well with 30–50 PAR at the substrate, and the medium-light plants benefit from the CO2 supplementation that supports the faster growth. The right light for the medium-light planted tank is a medium-intensity light (a 50–80 watt LED fixture over a 20-gallon tank), with a photoperiod of 7–9 hours per day.

The high-light plants. The demanding carpeting plants (Hemianthus callitrichoides, Glossostigma elatinoides, Eleocharis acicularis), the red plants (Ludwigia senegalensis, Rotala wallichii, Eriocaulon cinereum), and the demanding stem plants. The high-light plants do well with 50–100+ PAR at the substrate, and the high-light plants require the CO2 supplementation, the high-quality substrate, the regular fertilization, and the consistent maintenance. The right light for the high-light planted tank is a high-intensity light (a 100+ watt LED fixture over a 20-gallon tank), with a photoperiod of 8–10 hours per day.

The owner of a planted tank should choose the plants based on the lighting they are willing to provide. The owner who chooses the high-light plants without the high-light setup is the owner who is producing the algae farm. The owner who chooses the plants that match the lighting is the owner who has a thriving planted tank.

The four mistakes that turn a planted tank into an algae farm

Mistake 1: Too much light for too long. The most common mistake. The owner installs the brightest LED fixture available, the owner runs the light 10–12 hours per day, and the algae blooms within 2–4 weeks. The fix is to reduce the photoperiod to 6–8 hours per day, to reduce the intensity (the modern LED fixtures are dimmable), and to observe the plant and the algae response over the next 1–2 weeks.

Mistake 2: The wrong spectrum. The second most common mistake. The owner installs a light that is too blue (the all-blue "reef" light, sometimes inappropriately sold for planted tanks), or too red (the all-red "plant" light, sometimes sold as a supplement). The wrong spectrum produces the algae, and the wrong spectrum does not support the plant growth. The fix is to use a full-spectrum light (6500–7500K, or a multi-channel white + red + blue LED).

Mistake 3: Adding the high-light plants without the high-light setup. The third mistake. The owner adds the demanding carpeting plants or the red plants to a low-light setup, and the plants do not grow (because the light is insufficient), and the algae blooms (because the nutrients are not being used by the plants). The fix is to match the plant selection to the lighting, or to upgrade the lighting to support the desired plants.

Mistake 4: Not adjusting the lighting as the plants grow. The fourth mistake. The planted tank is established, the plants are growing, the plant mass is increasing, and the light is no longer appropriate for the increased plant mass (the plants are absorbing the light that the algae would have absorbed). The fix is to observe the plant growth, and to adjust the lighting (the photoperiod, the intensity) as the plant mass changes. The owner who is not adjusting is the owner who is producing the algae blooms or the plant stunting as the plant mass changes.

The bottom line

The lighting is the foundation of the planted tank, and the lighting is the variable that the owner can adjust most easily. The right photoperiod is 6–10 hours per day, depending on the plant selection. The right spectrum is the full spectrum (6500–7500K, or the multi-channel white + red + blue LED). The right intensity is 15–100+ PAR at the substrate, depending on the plant selection. The right equipment is the modern LED fixture, with the T5 fluorescent as the older standard and the metal halide as the high-end option.

The owner who matches the lighting to the plants is the owner who has a thriving planted tank. The owner who does not match is the owner who has an algae farm. The difference is the matching. The matching is the framework above. The framework, applied with observation, is the right prevention.

The planted tank is a rewarding hobby, and the planted tank that is properly lit is a tank that is beautiful, that is stable, and that is a source of pride for the owner. The planted tank that is poorly lit is a tank that is a source of frustration. The difference is the lighting. The lighting is the framework above. The framework, applied with observation, is the right prevention.

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About the author

Kenji Tanaka is a pond and aquarium keeper with 20+ years of experience ranging from indoor reef tanks to multi-thousand-gallon koi ponds, and including a wide range of freshwater setups, from simple goldfish tanks to high-tech planted tanks. He has worked with both hobbyists and small commercial aquaculture operations on lighting, plant selection, CO2 supplementation, and long-term system design, with a particular focus on the practical realities of maintaining a thriving planted tank. He writes regularly on the day-to-day realities of keeping the aquatic ecosystem stable and the plants healthy.