PAR Levels for Corals: Targets by Type (With a Cheap-Meter Plan)

PAR is the single most misunderstood number in reefkeeping, and the confusion costs corals constantly. Reefers obsess over alkalinity to the second decimal while running their lights at "looks bright enough" — then wonder why the acro browned out or the zoa bleached. This guide fixes that. It explains what PAR actually measures, gives you concrete targets by coral type, and lays out a plan to measure your own tank without buying a four-hundred-dollar meter. Light is not the parameter to guess at.

What PAR actually is

PAR — Photosynthetically Active Radiation — measures the amount of light in the 400-700nm band that corals' zooxanthellae can use for photosynthesis, expressed in micromoles per square meter per second (µmol/m²/s), universally shortened to just "PAR" in this hobby. Two things it is not:

• Not brightness to your eye. Human vision peaks in green-yellow; corals use heavily into the blue and red. A blue-dominant reef light can look dim while delivering high PAR, and a white-heavy light can look blazing while wasting output on wavelengths corals barely use.
• Not a fixture spec. PAR at the coral depends on the fixture, its intensity setting, mounting height, water depth, water clarity, and horizontal position under the light. The same LED reads 300 PAR at the surface and 80 PAR in a shaded bottom corner. PAR is a location, not a product.

There is a companion concept worth knowing: PUR (Photosynthetically Usable Radiation), which weights PAR by how usable each wavelength actually is to coral symbionts. You cannot easily measure PUR at home, but it explains why spectrum matters and why two fixtures reading identical PAR can grow corals differently.

Target PAR by coral type

These are working home-reef targets — ranges, not knife edges, because acclimation history matters as much as the number:

• Mushrooms, most zoanthids, Discosoma: 30-100 PAR. Low-light corals; high PAR bleaches or browns them.
• Soft corals (leathers, GSP, xenia, Kenya tree): 50-150 PAR. Broadly tolerant.
• LPS (hammer, torch, frogspawn, Duncan, candy cane, acan, blasto): 75-150 PAR. Moderate; many LPS burn above ~200.
• Montipora and other "easy" SPS (birdsnest, stylophora): 150-250 PAR. The SPS on-ramp.
• Acropora and demanding SPS: 250-400+ PAR. High light, and only after full acclimation.

Two rules override every number above. First, acclimate — a coral grown at 100 PAR moved to 250 will bleach regardless of whether 250 is "correct" for the species; step up over three to four weeks. Second, watch the coral, not the meter — browning means too little light (or too many nutrients), pastel or bleaching means too much. The meter tells you where you are; the coral tells you where it wants to be.

Spectrum: the other half nobody measures

PAR quantity without spectrum quality is half the picture. Reef corals evolved under blue-shifted light, and blue-heavy spectrums drive coral fluorescence and photosynthesis efficiently while suppressing nuisance algae. This is why reef keepers run "all blue" or 15-20K-appearance light rather than the daylight-white spectrum that grows more algae per PAR. A quality full-spectrum reef LED with independent channels lets you tune blue-dominant spectrum and intensity separately — the ideal combination for hitting both a PAR target and a usable spectrum. Whether LED or T5 serves your tank better is its own debate, covered in our LED vs. T5 comparison.

The cheap-meter plan

A dedicated reef PAR meter (the Apogee-class instruments) is genuinely the gold standard and genuinely expensive — hard to justify for a single tank. Here is the tiered plan real hobbyists use:

Tier 1 — borrow. Many reef clubs own a PAR meter that members borrow for a weekend, and some fish stores rent or lend them. One afternoon of mapping your tank's PAR at every rock position gives you a permanent map — light output changes slowly, so a single survey stays useful for a year or more.

Tier 2 — the phone app method. Lux-measuring phone apps paired with a rough lux-to-PAR conversion get you into the right ballpark for the price of nothing. The catch is real: conversion factors vary wildly by spectrum, so a blue-heavy reef light throws the numbers off significantly. Treat app readings as relative, not absolute — excellent for comparing your tank's bright and dim zones to each other, unreliable for a true PAR figure. Still, knowing that your top rock reads triple your sandbed is genuinely actionable.

Tier 3 — inexpensive standalone meters. Budget quantum meters exist at a fraction of the flagship price. They are less accurate, particularly under narrow-band blue light, but for setting relative zones and catching gross errors they earn their keep.

Tier 4 — manufacturer PAR charts. Most reputable LED makers publish PAR-at-depth maps for their fixtures at given heights and intensities. Combined with knowing your mounting height, these get you a defensible starting estimate for free — then you place corals conservatively low and adjust by observation.

Whatever tier you use, the workflow is the same: map your zones once, place each coral in the zone matching its target, start new corals at the low end of their range, and let coral behavior fine-tune from there.

Reading your corals as living light meters

Corals report their light situation continuously if you know the signs:

• Browning: too little light, or excess nutrients letting zooxanthellae multiply. Move up or brighten.
• Fading to pastel or white on the tops: too much light. Move down or dim.
• Stretching, elongating polyps reaching upward: light-starved, hunting for photons.
• Great color, good extension, steady growth: you found the spot. Leave it alone.

This feedback loop is why obsessive PAR precision matters less than people fear. Get within the right zone with a rough measurement, then let the corals guide the last adjustments over weeks.

Photoperiod and ramping

PAR targets assume a sensible photoperiod: most reefs run 8-10 hours of usable light with gentle sunrise and sunset ramps. Longer is not more light — it is more algae fuel. Ramping up and down over 30-60 minutes prevents the light-shock stress that instant on/off causes, and it lets fish and corals transition naturally. A blast of full intensity the moment lights snap on is a small daily stress you can eliminate for free with a schedule.

The acclimation protocol, step by step

More corals die from bad light transitions than from wrong PAR targets, so this deserves a concrete procedure rather than the vague "go slow" everyone repeats. When a new coral arrives, it has been living under whatever the seller ran — often much dimmer than your display, and you rarely know the exact number. So you assume low and climb. Place the new coral on the sandbed or the lowest shaded rock, well below its eventual target zone, for the first several days. Then move it up the rockwork in stages every four to seven days, watching for any pale-out or tissue stress at each stop, until it reaches the zone matching its type. If your fixture supports it, the cleaner method is leaving the coral in its final spot and ramping the fixture's intensity instead: start the affected channel at 40-50% of your normal setting and step it up 10% every few days over three to four weeks. Programmable LEDs make this trivial — many include a built-in acclimation mode that does the climb automatically. A modest nano fixture handles this just as capably as a flagship: a well-chosen nano reef LED with dimming covers a softie-and-LPS tank and lets you run the same gentle ramp. The principle underneath all of it is that zooxanthellae adapt to light on a timescale of weeks, and any change that outpaces that adaptation shows up as bleaching. Patience here is not caution for its own sake — it is matching your speed to the coral's biology.

FAQ

Do I really need to measure PAR?

Not with lab precision, but you should know your zones. The difference between a guessed setup and a mapped one is the difference between placing a torch where it thrives and cooking it on the top rock. Borrow a meter once; the map lasts for years.

Can I use a lux meter or phone app instead of a PAR meter?

For relative comparisons within your tank, yes — they reliably tell you which spots are brighter. For an absolute PAR number, no; spectrum throws lux-to-PAR conversions off badly under blue reef light. Use them to find your bright and dim zones, not to certify a species target.

My PAR is correct but the coral is unhappy — why?

PAR is one variable. Check acclimation history (was it moved too fast?), spectrum (enough usable blue?), flow, and nutrients. A "correct" PAR number on a coral that was slammed from 80 to 250 overnight still bleaches — the transition matters as much as the destination.

Light is a parameter, not a vibe. Map your zones, match corals to them, acclimate every addition, and read the corals as they tell you what they want. For where lighting fits in the complete husbandry picture, see our guide to coral care and propagation.