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Coral Reef Lighting Guide

Practical tipps to use Photone for coral lighting

Coral Reef Lighting Guide

In order to grow healthy and vivid corals, it is crucial to provide your reef tank with the proper spectrum, intensity and duration of light. Measuring photosynthetically active radiation (PAR) is crucial in assessing the quantity and quality of light for plants. PAR is not only the most important measure of light in terrestrial photobiology research for plants and crops, but also for underwater plants such as corals and other reef plants.

Reef Lighting

The entire reef ecosystem depends on light for photosynthesis, which is the process where corals derive 80-85% of their energy from. Furthermore, reef lighting stimulates the corals' chromoproteins that are responsible for creating beautifully vivid and vibrant colors within your reef aquarium.

Healthy corals showing vibrant and vivid colors

Color: The Light's Spectrum

As most corals originate from deeper waters where most yellow and red light is filtered by the water, they usually grow and look best under light with a blue coloration. The light color that a lamp produces is measured as the correlated color temperature (CCT) in Kelvin. The higher the Kelvin value, the more blueish the light gets. It is common for reef aquariums to use lighting ranging from around 6500K up to 20000K.

PAR Level Recommendations

Providing your corals the proper PAR / PPFD levels are crucial in order to keep them growing healthy.

Corals PAR Level (PPFD)
Acropora 200 – 600
Smaller Polyp Stony (SPS) 200 – 400
Larger Polyp Stony (LPS) 50 – 150

As corals are very resilient to different light levels, individual types are capable of adapting to a wide variety of light levels given enough time for acclimation to changes in intensities. Only focusing on PAR values might be dangerous as corals require careful observation and adjustments based on their individual needs and reactions.

Mind that PAR levels are measured as photosynthetic photon flux density (PPFD) in the unit of µmol/m²s.

Ensuring the Proper Photoperiod

As corals are photoperiodic – meaning that they respond to the relative lengths of light and dark periods – it is crucial to maintain proper, natural lighting times. If you run with higher PAR values, a good starting point is to limit the photoperiod to about 9 hours. If your aquarium runs at the lower range of PAR levels, extending the light period to up to 12 hours is probably better.

Signs For Too Much Light

Losing Color

If your corals start losing color, you may want to reduce the light intensity or reposition them to a lower or outward section of your reef aquarium and carefully observe how the corals react. Turning pale or going completely white is the most common reaction to too much light.

Reducing Size

Another sign of too much light is that the coral itself is shriveling instead of keeping or extending its size. This might or might not happen in conjunction with a loss of color as well.

Signs For Too Little Light

Turning Brown

If your corals start to turn brown, this might be a sign that they require higher light intensities. Mind that this is also a common reaction to suboptimal water quality that needs to be observed as well.

Slow Growth

If your coral is not growing as you expected, this might be a symptom of too little light or also way too much light. In this case it might be best to measure detailed light intensities and research for others' lighting experience what your specific coral.

Using Our App For Underwater PAR Measurements

There is absolutely no issue in using our Grow Light Meter app under water – it just needs a bit of preparation and some adjustments.

Without correction, PAR meters calibrated in air provide only relative values underwater as more radiation is backscattered out of the measurement element due to the refractive index of water being greater than for air resulting in a phenomenom called the immersion effect. Fortunately, our app offers a handy calibration feature that allows to counteract this phenomenom by applying an immersion effect correction factor of 1.32 to the air-calibrated measurement.


One obvious step is to prepare the smartphone and the eventual diffuser on iOS for its submerged usage. Even though most modern smartphones are water-proof to some extent, we recommend putting the smartphone in a clear plastic bag.

Putting the smartphone in a clear bag ensures that our whole setup is waterproof

Having some kind of translucent plastic in front of your camera will inevitably block some light and therefore require calibration to counteract the loss in transmission.


Calibration is not only required to to compensate the water-proof setup but also to factor in the physical changes in light transmission that is different in air and water. Therefore, we apply the immersion effect correction factor for accurate PAR measurements underwater with our air-calibrated Grow Light Meter.

As a first step, we need to calibrate for our waterproof setup. This requires us to take a reference measurement without the plastic bag that we'll then use within the calibration flow.

Taking the initial measurement under stable conditions
The same conditions but now with the clear plastic bag over the camera

As for the calibration, we'll use the "Light- / PAR-Meter" option and input our initial measurement as the reference meter's measurement:

The three broad steps to counteract the plastic bag in front of the camera

So far we've now successfully compensated for the plastic bag's transmission loss and should measure exactly the same with the bag as without the bag before.

As the second and final step, we need to apply our immersion effect correction factor of 1.32 to the measurement. We can again use the regular "Light- / PAR-Meter" calibration option to do so:

A second calibration is required to apply the immersion effect correction factor

A little hack is required: At the third step, input a pseudo measurement to get to the "Detected Peak Light" at the fourth step that is otherwise locked. After taking this measurement, you need to multiply it by 1.32 and input this result as the reference in step three that you can access by swiping back.

The broad three steps to factor in the immersion effect correction factor

Then press "Calibrate" and you're done: You can now use our Grow Light Meter as an underwater PAR meter.

Bonus Tips

Watch your corals over time to learn how to better recognize their needs
Measure repeatedly and regularly
If your reef ecosystem seems healthy, let it be

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Coral Reef Lighting Guide