What is PAR? Lighting & Brightness Explained for Planted Tanks

There are tons of different types of aquarium lights available to hobbyists, but how do you know which one to choose for your aquarium? One important question to ask is whether or not the light source is bright enough to grow the aquarium plants you’re looking to get. Let’s talk about PAR as a helpful determining factor when choosing your next planted tank light.

Defining PAR

PAR is simply an abbreviation for Photosynthetically Active Radiation (sometimes referred to as Photosynthetically Available Radiation). It quantifies the brightness levels of light that are used by plants in order to grow. If you have heard of lumens, this is a similar concept. The difference is that lumens describes the brightness of light that humans can see, whereas PAR describes the brightness of light that plants can use to grow. Specifically, PAR measures the number of photons (that have light wavelengths between 400–700 nm which plants can use to photosynthesize) that hit a 1 square meter surface every second. This type of PAR measurement is called the Photosynthetic Photon Flux Density (PPFD) and is measured in units of μmol/m²/s (which often shortened to “micromols” or just "PAR" amongst aquarists).

A simple way of looking at it is by considering the distance between the light source and the plant itself. When the light is closer to the plant (such as in a shallow aquarium), the PAR will be higher since more light is available to the plant. When the light is further away from the plant (such as in deeper aquarium), there is less light shining on the plant.

Photosynthesis simplified

Why PAR Matters for Aquatic Plants and Planted Tanks

Understanding PAR can be very helpful if you are trying to grow different types of aquarium plants, reduce algae in your aquarium, or compare different types of lighting models to determine which you need for your setup.

Each aquarium plant species has unique needs for lighting and fertilizer, and keeping them under the correct conditions can make a big difference in the health of the plant. In the aquarium market, plants are typically categorized as requiring low, medium, or high light to thrive. This rough guide translates those terms into PAR ranges:

• Low lighting: 10–20 PAR
  This would be an ideal setup for most low tech plants like anubias and cryptocoryne plants that still thrive even under very little light. Fertilizer will still be required but at minimal levels. High tech equipment like CO₂ (carbon dioxide) injection is not required. Plants will grow slowly but so will algae, making maintenance easier in the long run.

• Medium lighting: 20–35 PAR
  Most aquarium plants will do well in this range except for highly demanding plants. Fertilizing is essential in order for the plants to thrive since this setup has slightly higher lighting than low tech or low light aquariums. CO₂ is optional but not required. Plants will grow faster, but algae can also pop up faster as well. Regular maintenance and fertilization will be important to ensure the plants thrive and algae doesn’t grow out of control.

• High lighting: 40–50+ PAR
  Highly demanding plants and carpeting plants do best in high lighting. CO₂ is typically a necessity for such high levels of light, or else algae can take over. A rigid fertilization schedule will also be necessary to match the fast growth rate of the plants. If you wish to grow the most vibrant, red-colored plants, set up a high tech aquarium with strong lighting. 

If you have medium to high lighting and still wish to grow low light plants, you might not have to adjust your light. Consider keeping the high light plants closest to the light and in the brightest spots in the aquarium. Plant the low light plants near the substrate or in shaded areas away from direct lighting. This is because low light, slow growing plants tend to accumulate algae on their leaves when constantly exposed to bright light.

When comparing various models of aquarium lights, consider which plants you are trying to grow. Do they require high light or low light to thrive? When in doubt, go for a brighter unit that is also dimmable so that you can easily adjust it for different planted tank builds. Investing in a good light now will likely save you money in the future and ensure you have the capability to grow any plant you want, no matter what its light requirements are.

Bright lights shining on a planted aquarium

Is PAR Important for a Successful Planted Tank?

PAR and brightness are important factors in the success of a planted tank. Inadequate lighting can cause plants to suffer, and too much light can cause excessive amounts of algae growth. However, it is less important to chase specific PAR values and more important to understand the specific needs of your plants and observe their growth over time to make sure they are healthy. Besides the lighting, other important factors in the success of your planted tank include the fertilizer, substrate, and CO₂ levels. Even so, many aquatic plants are hardy and will adapt to a wide range of conditions. Understanding PAR is just one part of the whole picture of keeping an aquarium.

As a side note, many things can affect the PAR value or amount of photons that travel from your aquarium light and actually reach the aquarium plants. For example:

• Aquarium lids (e.g., greenhouse panels allow more light through than glass)
• Ripples in the water from surface agitation
• Depth of the fish tank
• Different colored backgrounds
• Light or dark substrates
• Tannins in the water
• Shadows from plants and decor
• Floating plants or tall plants (like vallisneria) at the surface

Measuring PAR on the Easy Plant LED

The best way to measure the PAR levels in an aquarium is to use a PAR meter. We set up an experiment to test the PAR output levels from the Aquarium Co-Op Easy Plant LED using a 65-gallon aquarium.

Detailed PAR chart for Easy Plant LED

As you can see, the PAR is the greatest at the center of the tank right underneath the light. The farther down you go in the tank or towards the far corners of the tank, the lower the available PAR becomes. This information can help inform you as to where to put the high light versus low light plants in your aquarium.

Of course, these PAR values were taken when the light was set at 100% brightness. To accommodate any low, medium, or high light plants you wish to grow, the Easy Plant LED has dimmable light settings that allow you to easily adjust the brightness from 10–100%, making it a great, all-purpose light solution that produces excellent plant growth..

Having run hundreds of aquarium lights at the Aquarium Co-Op fish store and on our own personal aquariums, we decided to make our own planted tank light to address all the pain points we’ve seen. The Easy Plant LED is ultra durable, energy efficient, and highly water resistant (in case you accidentally bump it into the tank). Plus, it has an extra-long 12-foot power cord to reach even the most faraway power outlet. For more details, check out the product page with hundreds of reviews written by actual customers.