What is PAR and Why is it Important?

PAR stands for Photosynthetically Active Radiation, which is simply a measurement of usable light intensity.

In a reef tank, that usable light fuels the zooxanthellae inside your corals. These microscopic algae provide most of the coral’s energy through photosynthesis. Too little light can lead to dull color and slow growth. Too much can cause stress or bleaching.

Why Should You Use a PAR Meter?

Because light fixture mounting height, rockwork, water depth, and even water clarity all affect light intensity, guessing when setting up your lighting usually isn’t a good path towards success. A PAR meter gives you real data so you can set your lights with confidence instead of trial and error.

How to Measure PAR in Your Reef Tank

Fortunately, testing PAR is incredibly simple. You just need the right tools.

For this guide, we’ll be using the Apogee MQ-510 PAR Meter, but the overall process remains the same regardless of which PAR meter you’re using. Just make sure your meter is designed to detect light in the typical reef tank spectrum range, usually around 450 nanometers. Otherwise, the intensity readings in your tank may be slightly higher or lower than what the meter reports.

Step 1: Attach The PAR Meter Sensor to a Wand

This isn’t technically required, but it makes testing PAR much easier. If you’re in a pinch, you can hold the sensor with your hand.

Most PAR meters have a purpose-built wand for the sensor, such as the Apogee AM-330 Telescoping Wand designed for Apogee PAR meters.

Step 1 placeholder image

Step 2: Place the Sensor in the Aquarium and Start Taking Readings

You’ll notice the number bounce around a bit, especially if your return pump or powerheads are running. Simply take an average of the reading.

For example, if it’s fluctuating between 120 and 130, mark that spot down as 125 PAR. A difference of 10 PAR in either direction will not meaningfully impact your coral-keeping success.

Important: The Apogee PAR Meter sensor is waterproof, but the handheld meter is NOT, so keep the meter safely away from the water.

Step 1 placeholder image

Step 3: Record Your Tank's PAR Readings

Measure at multiple depths, since PAR values drop the farther you move away from the light source.

We recommend taking readings at the sandbed, halfway up the rockwork, and at the top of the rockwork.

Tip: Print a photo of your reef tank and write the PAR values directly on the image for each area of the rockwork. This makes future adjustments much easier.

Step 1 placeholder image

Ready to Set Your Reef Lighting up the Right Way?

It's not a secret; quality PAR meters are expensive!

Consider borrowing a PAR meter from a local reef club or trusted hobbyist. Many local fish stores also offer rental programs, which can be a cost-effective way to map your tank.

Alternatively, we offer a unique 60-day return option on the MQ-510 PAR Meter, even if you use it:

  1. Purchase the MQ-510 PAR Meter
  2. Use it to test PAR and map out your tank's lighting
  3. Return it for a refund, minus a $100 restocking fee

On the other hand, if it fits within your reefing budget, owning a PAR meter long term gives you constant access to the best tool for fine-tuning your lighting. As your tank grows and corals change, you can make precise adjustments with confidence.

Plus, you can lend it out to fellow hobbyists and help them dial in their lighting. Accurate PAR readings can mean the difference between stressed corals and thriving reefs. By sharing the tool, you’re helping to protect corals and are supporting long-term success in your local reefing community.

MQ-510 Full Spectrum Underwater LED PAR Meter (OPEN BOX)
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MQ-510 Full Spectrum Underwater LED PAR Meter
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AM-330 Telescoping PAR Meter Wand
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MQ-210X Original Underwater Quantum PAR Meter
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USB Smart Quantum SQ-420X PAR Sensor
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SQ-520 Full Spectrum Smart Quantum LED PAR Sensor
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Frequently Asked Questions

What is PAR?

PAR is an acronym for "Photosynthetic Active Radiation." To put it simply, it's a measurement of light intensity.

Why is PAR Important?

In a reef aquarium, PAR matters because photosynthetic corals contain symbiotic algae called zooxanthellae. These algae provide the majority of the coral’s energy through photosynthesis.

If the light intensity is too low, corals can lose color and struggle to grow. If it’s too high, they can become stressed or even bleach. Supplying the right PAR range helps maintain healthy tissue, strong coloration, and steady growth.

Why Doesn't PAR Tell the Whole Story?

PAR measures light intensity, but it does not measure spectrum.

Corals rely heavily on specific wavelengths of light, especially in the blue range commonly used reef aquariums. Two lights can produce the same PAR reading but have very different spectral output, which can affect coral coloration and growth.

In short, PAR tells you how much light is available. Spectrum determines how useful that light is to your corals.

What PAR Values are Good for a Reef Tank?

Different corals require different light intensity, so there is no one-size-fits-all solution for every tank. This is one of the reasons why mixed reef tanks with various coral types are so difficult to perfect.

As a general rule of thumb, here are some ranges you can shoot for:

  • Soft Coral: 75-150 PAR
    "Softies" are generally the most forgiving when it comes to light and thrive in low to medium light intensities.
  • LPS Coral: 150-250 PAR
    Large Polyp Stoney (LPS) corals often do best in the lower end of this medium range. There are a handful of LPS that prefer lower light, so it's important to research your individual corals before deciding on a goal PAR value.
  • SPS Coral: 250-350+ PAR
    Small Polyp Stoney (SPS) corals are typically the most light-intensive corals in the hobby. In general, these corals like high light intensities, which best replicate their natural habitat on high-energy reefs.

Tip: Always start with a light intensity a little lower than you think you might need. Corals are much better at adapting quickly to lower light vs higher light and will often bleach if the light intensity is too high by a significant enough amount.

What is the Best Lighting Schedule?

We recommend starting out with the 1-8-1 lighting schedule:

  • 1 hour ramp up
  • 8 hours at steady intensity
  • 1 hour ramp down

You can schedule the lights to start ramping up so that they’re on when you’re most often home. For example, many reefers won’t have their lights come on until 11AM so they can enjoy the aquarium in the evening.