How to Choose Reef Lighting: PAR & Spectrum Explained

Every reef lighting argument eventually arrives at the same three letters: PAR. It's the number that turns "is my light good enough?" from a brand debate into a measurement — and once you understand it, most lighting confusion dissolves. This guide explains what PAR actually measures, the intensity bands different corals want, how spectrum fits in, and how to get real numbers over your own tank without guessing.

What PAR is (and what it isn't)

PAR — photosynthetically active radiation — is the portion of light usable for photosynthesis, and in practice reefers use the term for PPFD: the density of those photons landing on a surface, measured in micromoles per square meter per second (µmol/m²/s). It answers the only question a coral cares about: how much usable light arrives *here*. What PAR doesn't measure is quality — two lights can deliver identical PAR with very different spectra — and it says nothing about watts, lumens, or brightness to human eyes. Lumens are weighted to human vision and nearly useless for corals; watts measure electricity, not photons. When comparing fixtures or positioning corals, PAR at depth is the number; everything else is marketing adjacent.

The intensity bands corals actually want

Broad, well-established ranges — treat them as zones, not targets to hit exactly:

• Low (roughly 50–100 µmol): mushrooms, many zoanthids, leathers and other soft corals, and most non-photosynthetic-adjacent low-light species. Also the correct starting zone for acclimating almost anything new.
• Medium (roughly 100–200 µmol): the LPS heartland — euphyllia (hammers, torches, frogspawn), favias, acans, and most of the brain corals. Much of a mixed reef lives here happily forever.
• High (roughly 200–350+ µmol): SPS territory — acropora especially, along with montipora and the other small-polyp stony corals, plus maxima and crocea clams. Diminishing and eventually harmful returns beyond this band for most systems.

Two principles matter more than any specific number. First, stability beats intensity: a coral held at a steady 150 outgrows one bounced between 100 and 250 by an impatient hand on the app. Second, corals adapt — slowly. A colony can migrate between adjacent bands over weeks of gradual change; it bleaches when moved between them in an afternoon.

The tank is a PAR map, not a PAR number

Intensity in a reef varies several-fold from the water's surface to the sand and from directly under the fixture to the corners. That's not a flaw — it's the placement system. The rockwork's upper branches are your high-PAR real estate for SPS ambitions; mid-rock shelves suit the LPS band; shaded overhangs and the sandbed run low. One decent fixture over a scaped tank offers every band simultaneously, which is why coral placement, not fixture shopping, solves most "my coral isn't happy" lighting problems. When a coral sulks, moving it one zone down is free and usually right; browning tissue and stretched growth ask for more light, pale or bleached tips ask for less.

Depth compounds the map: water attenuates light meaningfully, so a fixture producing generous PAR just under the surface may land only the low band on the sand of a deep tank. This is why deep tanks push buyers up a fixture class and why mounting height is such an effective tuning tool — lowering a light raises the whole map, raising it flattens and widens the spread. Shallow frag tanks live at the opposite extreme, where modest fixtures deliver surprising intensity and the acclimation caution runs highest.

Spectrum: the other half

Coral photosynthesis runs on the symbiotic zooxanthellae's pigments, which absorb most strongly in blue wavelengths (with a secondary red interest that matters less at depth, where water filters red out anyway). This is why every serious reef fixture is built blue-dominant and why heavy blue looks "right" to experienced eyes — it's the light corals evolved under and the light that drives their fluorescent pigments to glow. Practical spectrum advice is short: run an established blue-weighted preset (the community's common AB+ style references exist precisely so nobody has to invent channel mixes), keep whites modest (they please human eyes, feed algae, and add little for coral), and stop adjusting. Spectrum stability is part of light stability. The chase for a magic slider combination has cost the hobby more coral than any cheap fixture ever did.

Getting real numbers: measuring PAR

You cannot judge PAR by eye — human vision auto-adjusts across light levels that differ by an order of magnitude. Options, in ascending accuracy: manufacturer PAR charts for your fixture at your mounting height (useful for setup, honest brands publish real data); borrowing or renting a quantum meter, which many local reef clubs and some stores offer precisely for this; or buying one — hobby-grade PAR meters have become genuinely affordable, and a group buy across a club amortizes a reference-grade unit. Measure at coral positions, not just the sand: map the rockwork tops, the shelves, and the shadows once, note the numbers, and lighting stops being a debate. Repeat after any fixture move, mounting change, or major coral growth — a thriving SPS colony is itself a shade structure changing the map beneath it.

Putting it together: a worked setup

A practical sequence for a new or re-lit tank. Mount the fixture at the manufacturer's recommended height and load a standard blue-weighted preset at conservative intensity — half of maximum or less. Map PAR at your intended coral positions; adjust mounting height (which shifts the whole map) or the master intensity (which scales it) until the rockwork spans roughly the low-through-high bands where you want them. Place corals into their bands, acclimating new arrivals a zone lower for a couple of weeks. Then raise intensity only in small monthly steps if growth goals demand it, re-checking the map as colonies expand. From that point the light is infrastructure: a fixed schedule, seasonal dusting of the lens, and placement decisions made against numbers instead of vibes. That's the entire discipline — and it's the difference between lighting as a recurring crisis and lighting as the most solved problem in the room.