How to Choose Planted Aquarium Lighting

Why shouldn't you choose a light by gallons?

Two tanks sold as the same volume can have very different shapes. A long, shallow tank spreads its water over a wide footprint close to the light, while a tall tank of the same volume puts the substrate much farther away. Tropica points out that the amount of light falls off considerably with distance from the light to the bottom of the tank, and from the center out toward the corners. That is why length and depth matter more than the volume printed on the box.

The table below lists typical footprints for common US glass tanks. These are common dimensions, not a standard: manufacturers vary by an inch or more, and rimless or "low-profile" versions often differ again. Always measure your own tank before buying a light.

Notice the 20 long and the 29: same length, so the same bracket size may fit both, but the 29 puts the substrate several inches deeper. A light that grows a carpet well in the 20 long may be noticeably weaker at the bottom of the 29.

Match the fixture to tank length

Most aquarium LED fixtures are sold for a range of tank lengths, either through extendable brackets or as fixed sizes. Check three things:

• The bracket range covers your exact length. If your tank is 30 inches, a fixture listed for 30 to 36 inches should fit; one listed for 24 to 30 inches may sit at its limit.
• The lit area reaches the ends. A fixture much shorter than the tank leaves the ends dim, which often shows up as slower, leggier growth at the ends.
• Width matters on deep-front tanks. On wide tanks such as a 40 breeder, a single narrow bar may not light the back row evenly. Some keepers use two fixtures or a wider panel.

How does water depth change the light you need?

Depth is where many purchases go wrong. Measure from the water surface to the substrate, not the glass height, since substrate and the gap below the rim reduce it. Then consider where your demanding plants will sit: carpeting plants live at the deepest point, while epiphytes tied to wood may sit halfway up.

If you are choosing between two fixtures for a deep tank, the one with more output and the ability to dim it down is usually the safer pick. You can always reduce intensity; you cannot add output a light does not have.

How much light do your plants need?

Your plant list and your CO2 plan should set the target, not the strongest light you can afford. Tropica gives a rough scale by its plant categories: about 10-20 lumens per liter of tank water for easy plants, 20-40 lumens per liter for medium plants, and more than 40 lumens per liter for advanced plants. Not every LED maker publishes lumens, so treat this as a sense of proportion rather than a spec to shop by.

• Low-tech tanks without injected CO2 usually do well with moderate light. Plants such as Anubias, Java fern, mosses, and many Cryptocoryne grow slowly and tolerate lower light. Strong light without enough CO2 and nutrients tends to favor algae over plants.
• CO2-injected tanks can use more light, because the plants have the carbon to keep up. Carpets and red stem plants often need this combination.

If you are not sure which path suits you, read our low-tech vs high-tech guide before buying. Matching light to the rest of the system matters more than any single number.

What is PAR, and how should you read PAR charts?

PAR stands for photosynthetically active radiation: the band of light, roughly 400 to 700 nanometers, that plants use for photosynthesis. In practice, the number on aquarium charts is usually PPFD, measured in micromoles of photons per square meter per second. Higher numbers mean more usable light reaching that spot.

A few cautions when reading manufacturer PAR charts:

• They are measured under specific conditions. Readings depend on mounting height, depth, whether the reading was taken in water or air, the meter used, and where under the fixture the sensor sat.
• They are not directly comparable between brands. One chart may show the peak reading directly under the center; another may show an average. Without matching conditions, comparing two charts number for number can mislead you.
• They describe the center, not the corners. Light usually drops toward the edges of the tank.

Treat PAR charts as a rough guide to how a fixture behaves at depth, not as a precise promise. Hobby sources publish general ranges for low, medium, and high light plants, but these vary between sources and are rules of thumb rather than hard targets.

Why watts per gallon is outdated

Watts per gallon was a rough rule of thumb from the era of fluorescent tubes, when most lamps converted power to light at broadly similar efficiencies. LEDs vary widely in efficiency and optics, so two fixtures drawing the same wattage can put very different amounts of light into the water. The rule also ignores tank shape, which is the main point above. Use length, depth, published output data, and dimming instead.

Color spectrum basics

Most planted tank LEDs combine white diodes with some red, green, and blue channels. Tropica recommends a color temperature of 5,500 to 6,500 K, close to daylight, and a color rendering index (CRI) of at least 90 so plants and fish look natural. Beyond that, spectrum is mostly about appearance: extra red can make plants look warmer and redder, while blue-heavy settings look cooler. Avoid assuming a particular spectrum will by itself fix growth problems. Intensity, duration, CO2, and nutrients have a much larger effect for most keepers.

Mounting options

• Rimless brackets clamp or rest on the top edge of rimless glass. Check the maximum glass thickness the bracket accepts.
• Rim legs sit on the plastic frame of a framed tank. These may not fit rimless glass at all.
• Hanging kits suspend the light from the ceiling or a stand arm. Raising a light spreads it wider but reduces intensity at the substrate.

Dimming and scheduling

A built-in controller that lets you dim the light and set a schedule is one of the most useful features for a planted tank. It lets you start a new tank gently, ramp up as plants establish, and add a gradual sunrise and sunset. If your fixture lacks scheduling, an external timer works fine for on and off. See our timers and lighting schedules guide for setup.

Selection checklist

• Measure the tank's exact length, width, and water depth.
• Decide on low-tech or CO2-injected before choosing output.
• List the plants you want and where they will sit in the tank.
• Confirm the bracket range and mounting style fit your tank.
• Prefer dimming and scheduling, built in or added.
• Read PAR charts as rough guides, noting how each was measured.
• Plan where the power supply and cord will run.

Quick answers

What size light do I need for a planted tank? Pick a fixture whose bracket range covers your tank's exact length, then check it has enough output to reach the substrate at your water depth. Gallon size alone is a poor guide.

What color temperature is best for aquarium plants? Tropica recommends 5,500 to 6,500 K, close to daylight. Beyond that, spectrum mostly changes how the tank looks.

Can I compare PAR numbers between brands? Not directly. PAR charts are measured at different heights, depths, and positions, so use them as a rough guide to how one fixture behaves at depth.

Is watts per gallon still useful? No. It dates from fluorescent tubes, and LEDs vary too much in efficiency for wattage to predict how much light reaches your plants.

Common mistakes

• Buying by gallon class and ending up with a light that is too short or too weak for a deep tank.
• Running a strong light at full power in a new low-tech tank, which often leads to algae.
• Comparing PAR numbers from different brands as if they were measured the same way.
• Choosing rim legs for a rimless tank, or a rimless bracket that does not fit the glass thickness.
• Expecting a spectrum setting to solve problems caused by too much light or too little CO2.

Next steps

Once you have a light, set its intensity and photoperiod with our light intensity and duration guide. If algae appears, the algae troubleshooting guide walks through likely causes. Browse fixtures by type in the lighting hub, or use the setup finder to match lighting with the rest of your system.

If you are ready to shop, our picks are sorted by tank size: lights for 10-gallon aquariums, lights for 20-gallon aquariums, and lights for 40-gallon aquariums. To see how two popular plant lights differ on tank-length fit, mounting, and control, read Fluval Plant 3.0 vs Finnex Planted+ 3.0, or the individual Fluval Plant 3.0 review and Finnex Planted+ 3.0 review.