PAR, The Only Lighting Metric That Actually Matters

PAR stands for Photosynthetically Active Radiation, the measurement of light
energy in the wavelength range (400–700 nanometers) that drives photosynthesis.
It’s the standard measurement used to evaluate reef tank lighting because it
directly predicts the light energy available to coral zooxanthellae.

Most other lighting metrics, watts, lumens, lux, are not useful for reef
keeping:

• Watts measures electricity consumption, not light output. A 50-watt fixture can produce more or less PAR than another 50-watt fixture depending on LED efficiency and optics.
• Lumens measures brightness as perceived by the human eye, weighted heavily toward green and yellow wavelengths that eyes are most sensitive to. Reef corals use blue and red wavelengths most heavily, which lumens undervalues.
• Lux is lumens per square meter, the same problem as lumens, just measured at a distance.

PAR is measured in μmol/m²/s (micromoles of photons per square meter per second).
It’s measured at a specific depth and position in the tank using a PAR meter, a specialized instrument that’s worth borrowing or renting for initial fixture
setup if you’re serious about getting placement and intensity right.

PAR Requirements by Coral Type

For a beginner reef focused on soft corals and LPS, you need a fixture capable
of delivering 75–200 PAR at coral placement depth. This is achievable with
most quality beginner LED fixtures, the AI Prime 16HD, for example, delivers
200+ PAR at 12 inches depth at moderate intensity settings, covering the full
beginner to intermediate coral range.

The important distinction: PAR at the fixture is not the same as PAR at the
coral. PAR falls off significantly with depth and with lateral distance from
the fixture center. A fixture that claims 300 PAR at 12 inches directly below
the center may deliver only 80 PAR at the bottom corners of the same tank.
This is why fixture placement, mounting height, and intensity settings all matter.

Spectrum, What It Is and Why It Matters

Spectrum refers to the wavelengths of light the fixture produces. Different
wavelengths have different effects on coral health, color expression, and
growth. Understanding the basics prevents two common beginner mistakes:
running too much blue (corals look great under blue light but are being
underfed photosynthetically) and running too much white (corals get adequate
PAR but colors fade toward brown).

Key Wavelengths for Reef Lighting

• Blue (420–480 nm), the most important wavelength for
  zooxanthellae photosynthesis. Deep blue (420–450 nm, sometimes called
  violet or actinic) drives the fluorescent pigments that produce the
  vivid coloration in many corals. Cyan blue (460–480 nm) is the primary
  photosynthetic driver for most coral species. Blue-dominant spectrum
  is what gives reef tanks their distinctive look.
• Red (620–700 nm), the other primary photosynthetic
  wavelength for the zooxanthellae chlorophyll pigments. Most reef LED
  fixtures produce relatively little red because it promotes green algae
  growth and is less visually appealing, but some red is necessary for
  complete photosynthetic function.
• White/full spectrum (400–700 nm), broad spectrum LEDs
  that cover the full PAR range. White LEDs produce the PAR that drives
  growth and calcification. A fixture that runs only blue channels looks
  dramatic but may be delivering less actual photosynthetically useful
  energy than its appearance suggests.
• UV/violet (360–420 nm), below the standard PAR range
  but present in quality reef fixtures. UV drives fluorescent protein
  expression in many corals, the blues, purples, and greens that make
  reef tanks visually distinctive. Not essential but meaningful for color.

Blue vs. White, The Practical Reality

A common beginner mistake: running the light mostly on blue channels because
the tank looks better that way. Blue-heavy lighting makes corals fluoresce
dramatically and is visually stunning, but if the white channels are turned
down significantly to make the tank “bluer,” the total PAR delivered to
corals may be insufficient for growth even if the fixture is capable of more.

The right approach: run the white and blue channels in the ratio recommended
by the fixture manufacturer for reef keeping (typically 40–60% blue, 20–40%
white at midday peak), then adjust for aesthetics within that general ratio.
Don’t sacrifice PAR delivery for appearance.

Photoperiod, Setting Up a Schedule That Works

The photoperiod is the lighting schedule, when the lights come on, how
bright they get, and when they turn off. A well-designed photoperiod
mimics the natural light cycle of a tropical reef, provides corals with
a consistent and predictable light environment, and minimizes algae growth
by avoiding excessive total daily light exposure.

The Recommended Beginner Photoperiod

For most beginner reef tanks, this schedule works well:

This gives a total photoperiod of approximately 12 hours with a 3-hour
midday peak. The dawn and dusk ramps are important, they prevent the
sudden on/off light change that startles fish and stresses corals adjusting
from darkness to full intensity.

During cycling and for the first 4–6 weeks of a new tank:
run this same schedule but cap all channels at 20–30% of the above values.
This suppresses algae growth during the maturation phase without eliminating
the light cycle entirely. Ramp up gradually over 4–6 weeks as the tank matures.

Total Daily Light Period (DLI)

DLI (Daily Light Integral) is the total amount of PAR delivered to a coral
over a full day, it’s the product of intensity and duration. This matters
because you can achieve the same DLI with either high intensity for a short
period or lower intensity for a longer period.

For most beginner reef corals, a DLI of 10–20 mol/m²/day is the target.
The schedule above, running at the indicated percentages, delivers
approximately this range for most quality beginner LED fixtures. If you
don’t have a PAR meter, using the manufacturer’s recommended intensity
settings for your tank size on this schedule is a reliable starting point.

Starting the Light in a New Tank

A new tank’s first enemy is algae, and algae’s primary fuel is excess light.
Running a reef light at full intensity from day one produces exactly the
environment algae thrives in: abundant light, elevated nutrients from cycling,
and no established predators or competition to keep algae in check.

The correct startup approach:

Weeks 1–4 (During Cycling)

• Run the full photoperiod schedule (7 AM – 7:30 PM) but at 15–20% of the intensities shown above
• This establishes the light cycle routine without fueling excessive algae
• Rock will still begin colonizing with coralline algae spores from the live rock, the low-intensity light doesn’t prevent this

Weeks 4–8 (Post-Cycle, Pre-Coral)

• Ramp up to 30–40% of target intensities
• Monitor for algae response, if algae accelerates significantly, hold at current intensity rather than continuing to increase
• This is the acclimation period for any future corals, you’re building toward conditions the corals will eventually live in

Weeks 8+ (With Corals)

• At the point of adding first corals, ramp intensity back slightly, to 25–30% of target, so the coral begins at the low end of its PAR range
• Increase intensity by 5–10% per week over 4–6 weeks until reaching target PAR for the coral’s species
• Watch coral behavior, full polyp extension and good color are signs the coral is adapting well. Closed polyps, pale color, or tissue recession indicate too much or too little light

Mounting Height and Fixture Placement

Mounting height determines the PAR distribution across the tank, both the
peak intensity at the center and how evenly the light spreads to the edges.
Lower mounting produces higher center PAR but worse edge coverage and more
shadowing. Higher mounting spreads light more evenly but reduces peak PAR.

General Starting Points by Fixture Type

Center the fixture over the tank. For tanks longer than 24 inches, a single
pendant fixture may not provide adequate edge coverage, check manufacturer
coverage ratings for your tank dimensions. A single AI Prime 16HD covers up
to approximately 24″ x 24″ at recommended mounting height and intensity.
Longer tanks typically require multiple fixtures or a bar-style LED.

Verifying PAR Without a Meter

If you don’t have access to a PAR meter, use coral behavior as a proxy:

• Correct PAR: Corals fully extend polyps during the photoperiod, show good color, and grow at a visible rate over weeks to months
• Too little PAR: Corals extend fully but colors fade toward brown over weeks; growth is slow or absent; corals may reach toward the light source
• Too much PAR: Corals retract during peak light hours; tips or upper surfaces bleach white or pale; some LPS corals deflate and fail to extend during midday

These are slow signals, changes develop over weeks, not days. This is why
acclimating corals to new light slowly is safer than starting at full intensity
and waiting to see if something goes wrong.

Acclimating Corals to a New or Changed Light

Corals that were living under different light, in a fish store, in a
friend’s tank, under your previous fixture, need time to adapt to new
lighting conditions. The zooxanthellae density inside coral tissue adjusts
over 2–4 weeks to match light intensity. Moving a coral abruptly from low
to high light damages the zooxanthellae and bleaches the coral. Moving from
high to low light typically just slows the coral down, less damaging but
still a stressor.

The Right Acclimation Approach

1. Place new corals at the lowest PAR position in the tank, typically the bottom, in a shaded spot, regardless of where they’ll eventually live
2. Leave them there for 1–2 weeks, observing daily. Full polyp extension within a few days is a good sign. Closed polyps or pale color at low light suggests acclimation stress, give more time
3. Move the coral up or to a higher-PAR position incrementally, one step per week, until reaching the target placement
4. At each new position, observe for 5–7 days before moving again
5. If bleaching begins at any point, pale coloration spreading from the tips or upper surface, move back to the lower position immediately and slow down the acclimation

This process takes 3–6 weeks for a coral moving from low to high PAR. It’s
frustrating to wait when you have a beautiful spot picked out, but a coral
that bleaches from light shock takes months to recover, if it recovers at all.

Acclimating to a New Fixture

When upgrading to a new, more powerful light fixture, acclimate all existing
corals exactly as above, start the new fixture at 25–30% intensity and
ramp up 5–10% per week. Don’t assume existing corals can handle the new
fixture at the settings you intend to use long-term. Every fixture upgrade
is a re-acclimation for every coral in the tank.

Common Beginner Lighting Mistakes

Running Lights Too Long

A 14–16 hour photoperiod doesn’t produce healthier corals, it produces more
algae. Coral photosynthesis saturates well within a 10–12 hour photoperiod.
Additional light hours primarily benefit algae, which has a lower light
saturation point than most corals and continues photosynthesizing at
lower intensities than corals find useful.

If you’re fighting persistent algae and your photoperiod is over 12 hours,
reduce to 10 hours and hold it there for 4–6 weeks before troubleshooting
further. This alone resolves many beginner algae problems without any
equipment changes.

Turning Lights On and Off Abruptly

Fish startle when lights go from off to full brightness instantly, a stress
response that suppresses immune function over time. Corals also prefer a
gradual transition. All quality reef controllers include dimming programs
for this reason. If your fixture has no dimming capability, it’s a meaningful
limitation for a reef tank.

Buying a Light Based on Color Appearance

The “purple glow” of some budget LED fixtures looks reef-like but delivers
almost no usable PAR. Pure actinic/UV fixtures produce stunning fluorescence
effects but don’t deliver the broad-spectrum PAR that drives coral growth.
Evaluate fixtures based on PAR output data at relevant depths, not on how
they look in a display tank.

Not Accounting for Tank Depth

A fixture that delivers 200 PAR at 6 inches depth may deliver only 80 PAR
at 18 inches depth in the same tank. If you’re keeping corals at the bottom
of a tall tank (20+ inches water depth), ensure the fixture is capable of
delivering adequate PAR at that depth, not just at the top of the water column.

Chasing Coral Color With Light Alone

Coral color is determined by a combination of light spectrum, light intensity,
water chemistry (particularly nitrate and phosphate balance), and genetic
expression. A coral that’s brown in good light probably has elevated nutrients
or inappropriate spectrum, not just insufficient blue. Adjusting light
without addressing water quality produces limited results.

Choosing the Right Light for Your Tank

Match the fixture to your tank size and coral goals, not the other way around.

See the full fixture comparison:
Best Reef Tank Lights for Beginners

What to Look for in Any Fixture

• Published PAR data at relevant depths and distances, not just “high PAR” claims without numbers
• Individual channel control, the ability to adjust blue and white channels independently to dial in spectrum and intensity separately
• Programmable schedule, timer and dimming capability built in or via controller/app. This is a basic requirement for a reef light, not an advanced feature
• Adequate coverage for tank footprint, verify the manufacturer’s coverage rating matches your tank’s dimensions, not just its volume
• Brand reputation for reliability, a fixture that fails six months in requires a full re-acclimation for every coral when the replacement arrives

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