PAR vs. PUR—Why I Focus on “Usable” Light for Coral Growth

In the world of reef keeping, we talk about PAR (Photosynthetically Active Radiation) constantly. We buy expensive meters and aim for specific numbers, but I’ve found that many hobbyists are missing a crucial piece of the puzzle: PUR (Photosynthetically Usable Radiation).

As I was designing the MegaReef 100, I realized that if we want to push the boundaries of coral growth and coloration, we have to stop chasing “raw power” and start chasing “usable energy.”

The Problem with PAR Alone

PAR measures all light between 400nm and 700nm. While it’s a great baseline, it doesn’t tell the whole story. To a PAR meter, a “watt” of green light and a “watt” of violet light are roughly the same. However, to your corals, they are worlds apart.

In my testing, I’ve seen tanks with “perfect” PAR readings struggle with growth because the spectrum was inefficient. This is where PUR comes in.

What is PUR?

PUR is the fraction of PAR that is actually absorbed by the zooxanthellae (the symbiotic algae inside your corals). Corals aren’t just “light eaters”—they are specific wavelength hunters.

• Chlorophyll a and c2: These are the primary engines of coral growth, and they have massive absorption peaks in the 420nm (Violet) and 450nm (Royal Blue) ranges.
• Accessory Pigments: These protect the coral and give them their incredible fluorescent colors. They crave the 405nm (UV/Violet) and 480nm (Cyan) wavelengths.

My Design Philosophy: Maximizing PUR

When I developed the 7-channel spectrum for the MegaReef 100, my goal was to maximize the PUR-to-PAR ratio. I didn’t want to waste energy on “filler” light that only serves to grow nuisance algae.

1. High-Efficiency Blue Engine: By focusing heavily on the 420nm–480nm range, I ensure that almost every photon hitting the water is “workable” for the coral.
2. Spectral Customization: Using the Real-Time Spectral Visualization in the AquaHorti app, I can see exactly where my energy is going. If I want to drive growth, I boost the 450nm; if I want color pop, I lean into the 405nm.
3. The Result: Higher growth rates at lower total wattages, which means less heat in the tank and a lower electricity bill.

The Takeaway

Don’t be fooled by a high PAR reading if the spectrum is “empty calories.” When you’re choosing your next light, ask yourself: “How much of this light is actually usable by my reef?”

I’ve spent hundreds of hours in the lab and in front of my own tanks to ensure that when you use an AquaHorti light, you aren’t just hitting a number—you’re feeding your reef exactly what it needs to thrive.