Reef lighting 101: PAR, spectrum, and what your corals actually need

The two numbers that matter

PAR (Photosynthetically Active Radiation) measures light intensity at the level your corals are. Spectrum describes the wavelengths your light produces.

Forget watts. Forget lumens. Watts measure power consumption, lumens measure light visible to humans, neither tells you anything about coral growth.

Target PAR by coral type

• Soft corals (zoas, mushrooms, GSP, Xenia): 50–150 PAR
• LPS (Duncans, hammers, torches, Acans): 75–200 PAR
• SPS (Acropora, Montipora, etc.): 200–400 PAR

Buy or borrow a PAR meter once. Apogee MQ-510 is the gold standard; Seneye is a budget alternative. You don't need to own one — borrow from a local club.

Spectrum

Reef LEDs lean blue-heavy because:

1. Coral pigments fluoresce more under blue light (the "pop" effect)
2. Photosynthetic zooxanthellae absorb blue/violet wavelengths efficiently

Most modern LED fixtures default to a "reef" spectrum that's roughly 60–70% blue/violet, 20–30% white, and 5–10% red/green. This is fine. Don't overthink it.

Acclimation

Never plug a new fixture in at full power. Corals from your local fish store have been under their lights, not yours. Start at 30–40% intensity and ramp up over 2–4 weeks. The damage from light shock is invisible and slow — you don't see it until polyps stop opening or a coral pales.

Photoperiod

A typical reef photoperiod runs 8–10 hours of "lighting" with peak intensity for ~4–6 of those hours. Most fixtures support gradual ramp-up and ramp-down — use it.