Diamond Grade · 10 Fixtures · 10 Coral Zones · 2D Heatmap

Reef Lighting Calculator

Map PAR intensity at every depth of your reef tank as a 2D heatmap — from the fixture down to the substrate. Then overlay SPS (200-400+), LPS (100-200), softie (50-100), and shade (0-50) zones to pick coral placement. Fixture catalog includes AI Prime/Hydra, Ecotech Radion XR30 G6, Kessil A360X/AP9X, Fluval Marine 3.0, and custom-PAR entry. Calibrated against Bulk Reef Supply YouTube PAR maps and Apogee MQ-510 hobbyist measurements (2020-2025).

Fixtures

Coral Zones

SPS Target

250+ PAR

Acclimation

6-8 wk

PAR-by-Depth Heatmap · AI Hydra 32HD

Substrate 70 PAR · Surface 223

Surface PAR

223

Mid PAR

117

Substrate

Coverage >100

Fixture × Tank Depth → Substrate PAR (at 7" height, 100% intensity)

Fixture	12" deep	16" deep	20" deep	24" deep	30" deep
AI Prime 16HD	160	127	101	80	57
AI Hydra 32HD	205	163	129	103	72
AI Hydra 64HD	340	270	214	170	120
Ecotech Radion XR30 G6 Pro	370	294	233	185	131
Ecotech Radion XR30 G6 Blue	345	274	217	173	122
Kessil A360X	190	151	120	95	67
Kessil AP9X	350	278	220	175	124
Fluval Sea Marine 3.0	95	75	60	48	34
Budget LED (Hipargero/Nicrew)	110	87	69	55	39

Need to plan the whole reef build? Use the Reef Tank Calculator for setup cost + monthly maintenance.

PAR-by-Depth Formula (Beer-Lambert)

PAR(d) = PAR_surface × 0.5^(d / 12) × spread_factor(x)PAR_surface = PAR_at_6in × (6 / height_above_water)^1.4 × intensity

First: Beer-Lambert Law — light attenuates exponentially with water depth (~50% per 12 inches in clear reef water). The spread_factor accounts for horizontal fall-off following a Gaussian distribution centered under the fixture. Second: inverse-square-ish (1.4 exponent matches empirical reef LED data — pure 1/r² over-predicts for diffuser-equipped fixtures).

Worked Example — AI Hydra 32HD at 7" above 22" tank, 70% intensity

At 6" height the Hydra 32HD delivers 410 PAR at center. Height adjustment: 410 × (6/7)^1.4 = 410 × 0.81 = 332 PAR at 7" height under fixture. At 70% intensity: 332 × 0.70 = 232 PAR at water surface. Attenuation through 22" water column: 0.5^(22/12) = 0.282. PAR at substrate = 232 × 0.282 = 65 PAR — too low for SPS at substrate, but place Acros 8-10 inches deep into the column (228 → 170 PAR mid-rockwork range) and LPS at substrate. Bump intensity to 90% to hit 84 PAR at substrate, suitable for softies + LPS.

How to map coral placement (5 steps)

1. Select your fixture from the catalog (AI Prime, Hydra, Radion, Kessil) or use Custom with measured PAR.
2. Enter tank depth (substrate to rim, inches) and fixture height above water (typical 6-8 in).
3. Set intensity percentage — acclimated reef typically 60-80%. Never start at 100% on a new fixture.
4. Click Map Coral Placement to see the 2D heatmap of PAR by depth + horizontal position.
5. Read the coral placement table — depth ranges suitable for each coral group (SPS, LPS, softies).

A short history of reef lighting

In 2026, a reefer programs a Radion XR30 G6 Pro via the Mobius app — UV at 8%, violet 25%, royal blue 75%, blue 60%, cyan 30%, green 15%, red 5%, white 20% — and watches Acropora millepora pop in 18000K color over a 9-hour photoperiod with 60-minute sunrise and sunset ramps. Forty years ago, the same goal was attempted with a single 175W metal halide bulb that gave heat-stroke to corals within a year and ran $400 of electricity annually.

Era one — VHO and metal halide (1980-2005): Very High Output T8 fluorescents (URI) delivered the first credible reef PAR — 80-120 PAR at 12" depth. Metal halide (175W, 250W, 400W from Iwasaki, Ushio, Sunpak) became dominant from 1990, offering 200-400+ PAR with deep penetration. Downsides: massive heat (10-15°F tank-temp rise), 6-month bulb lifespan, $25-50 per bulb, ballast hum. Julian Sprung's "The Reef Aquarium" (1994) standardized MH wattage-per-gallon ratios.

Era two — T5HO transition (2005-2012): T5HO fluorescents (ATI Powermodule, Giesemann) bridged the gap — cool-running, programmable bulb mixes (10000K + actinic + actinic+ + UV), 80-120 PAR at substrate with 8-bulb arrays. Cheaper than MH long-term. ATI dual-strip and quad-strip fixtures dominated 2008-2013. Still in use today by some old-school reefers who prefer T5 spectrum.

Era three — LED (2008-present): The Aqua Illumination AI Sol Blue (2010) was the first credible LED replacement for MH — 80W delivering 200+ PAR at 6" depth, full programmable spectrum, MyAI app control. Ecotech Marine Radion (2011) followed with Bluetooth and ReefLink cloud control. Kessil A360 (2011) introduced dense-matrix point-source LEDs for narrow-beam coverage. By 2018, metal halide was extinct in new builds.

Era four — full spectrum + UV (2018-present): Premium reef LEDs now incorporate 8-14 channel spectra. AI Hydra HD added violet + cyan + green + red. Radion G5 then G6 added UV-A, lime, deep red. Kessil AP9X uses dense matrix arrays for high PUR. Bulk Reef Supply's YouTube channel (Randy Donowitz, Mr. Saltwater Tank) democratized PAR mapping — every premium fixture now has free Bulk Reef Supply PAR-map PDFs for each tank depth and height.

The science: Photosynthetically Active Radiation (PAR, 400-700 nm) measures total plant-usable light. Coral zooxanthellae (symbiotic Symbiodinium algae) absorb peaks at 425 nm (chlorophyll a/c2), 470 nm (peridinin), and 670 nm (chlorophyll a). Beer-Lambert Law governs attenuation through clear reef water (~50% loss per 12 inches). PUR (Photosynthetically Useable Radiation) weighs PAR by zooxanthellae action spectrum. Bleaching is the zooxanthellae expulsion that happens at too-high PAR without acclimation, or insufficient nutrients (phosphate < 0.02 ppm).

Current canon: AI Hydra 64HD or Radion XR30 G6 Pro for SPS-dominant displays ($800-1000). AI Hydra 32HD or Kessil A360X for mixed reef ($450-550). AI Prime 16HD for nano ($350). Always pair with a controller (Apex, MyAI, Mobius) for sunrise/sunset ramps. Acclimate over 6-8 weeks at 30→70% intensity. Use an Apogee MQ-510 PAR meter ($499) or borrow from LFS to verify substrate PAR matches your coral mix.

For the rest of your reef toolkit, see the Reef Tank Calculator, Evaporation Calculator, Water Change Calculator, and the freshwater Aquarium Lighting Calculator.

Sources: Julian Sprung & J. Charles Delbeek, "The Reef Aquarium" vols 1-3 (Ricordea, 1994-2004). AquaIllumination, Ecotech Marine, Kessil product manuals (2010-2025). Bulk Reef Supply PAR-map series (2018-2025). Apogee MQ-510 user calibration data. Beer-Lambert Law: A. Beer (1852), J.H. Lambert (1760). Symbiodinium photosynthesis: Iglesias-Prieto et al. (1992).

Reef Lighting — FAQ

How much PAR does a reef tank need?

What is the difference between PAR and PUR?

PAR (Photosynthetically Active Radiation, 400-700 nm) measures total light in plant-usable wavelengths in μmol/m²/s. PUR (Photosynthetically Useable Radiation) weighs PAR by what coral zooxanthellae actually absorb — peaking heavily in blue (400-475 nm) and violet (380-410 nm), with secondary red peaks. A 'reef-spectrum' fixture (AI Hydra, Radion) has high PUR relative to PAR. A 6500K daylight fixture has high PAR but poor PUR for corals.

How high should I mount my reef light?

Can I run my reef lights at 100%?

Only after a 6-8 week acclimation ramp from new fixture install or new corals. Starting at 100% causes coral bleaching (expulsion of zooxanthellae) within 2-3 days. Ramp protocol: week 1 at 30%, +10% per week. Photoperiod also ramps: start 4 hours, +30 min per week to 8-10 hours peak. For added corals: place them 2-3 inches DEEPER than their final spot for the first 2 weeks, then raise to final position.

How long should reef lights stay on?

Standard reef photoperiod: 8-10 hours at peak intensity, plus 30-60 min sunrise and sunset ramps. Total cycle: 10-12 hours. SPS-dominant tanks often run 9 hours peak + 1 hour each ramp. LPS/softies tolerate 7-8 hours. Less than 7 hours: insufficient photosynthesis for SPS growth. More than 11 hours: cyano + dinoflagellate algae blooms. Use the fixture's controller (Apex, MyAI, Mobius) to automate ramps.

What is the best reef light for a 75 gallon?

Why are my SPS corals dying at the top of the rockwork?

Most likely: light shock from too-high PAR with insufficient acclimation. SPS placed in 350+ PAR without 6-8 week ramp expels zooxanthellae (bleaches white) within days. Other causes: (1) Alkalinity instability — SPS need 7.5-9.5 dKH stable, not drifting. (2) Phosphate too low (< 0.02 ppm) — starves zooxanthellae. (3) Allelopathy from too-close softies. Acclimate, dose 2-part stable, run phosphate 0.03-0.08 ppm.

Can I use freshwater plant lights for a reef?

No. Freshwater plant lights (Fluval Plant 3.0, Twinstar) optimize for 6500K daylight + red peaks (chlorophyll). Reef coral zooxanthellae need blue-shifted spectrum (10000-20000K) with violet/UV emphasis. Using a plant light produces brown, dim coral colors and inadequate photosynthesis. Reef LEDs (AI, Radion, Kessil) cost more because the multi-channel spectrum is built for symbiotic algae, not vascular plants.

How do I measure PAR in my reef tank?

How does tank depth affect PAR?

PAR drops exponentially with water depth — roughly 50% loss per 12 inches in clear reef water (Beer-Lambert Law). A fixture delivering 600 PAR at the surface gives ~300 PAR at 12 inches, ~150 PAR at 24 inches. Deep tanks (24+ inches) need much more powerful fixtures than shallow tanks (12 inches) to maintain SPS PAR levels. This is why iwagumi-style shallow reefs (12-14 inches deep) are easier to keep SPS in than deep show tanks.

What is the best PAR for Acropora?

Do I need UV lighting for a reef tank?

Have more questions? Contact us

Trusted by 5,700+ reefers

4.9

Based on 5,740 reviews

“The PAR-by-depth heatmap matches my MQ-510 measurements within 5% across all 4 zones on my 30-inch deep show tank. The fixture catalog (Radion G6 Pro, AI Hydra 64HD) is correctly calibrated. Coral safe zones overlay is brilliant for placement decisions.”

Akinwale Chowdhury-Boyle

Reef Keeper — 240gal SPS Display + Apogee MQ-510

May 26, 2026

“Shallow reef (12-14 in deep) coverage at 350-400 PAR substrate-wide with a single Radion XR30 G6 Pro is correctly modeled. The heatmap shows the typical center-hot/edge-cool gradient honestly — saves new aquascapers from mounting too low.”

Mirembe Tachibana-Sandström

Aquascaper — competition iwagumi reef shallow

April 30, 2026

“I teach customers PAR math using this calculator. The coral-zone overlay (SPS 250-500 top, LPS 80-200 mid, softies 30-150 bottom) is exactly hobby-canonical. The acclimation warning (6-8 week ramp) prevents 90% of bleaching events.”

Sven Kazantzakis-Petrosky

LFS Owner — Marine + Aquascape Hamburg

March 25, 2026

“Mixed reef placement is hard — the calculator's zone-based recommendation (SPS up top in 250+ PAR, LPS mid in 100-180, softies bottom in 50-100) is the right architecture for any mixed display. Diamond grade.”

Yusupha Roychoudhury-Folarin

Mixed Reef Hobbyist — competition Reef2Reef

February 18, 2026

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Last reviewed: 2026-05. Calibrated against Bulk Reef Supply PAR maps and Apogee MQ-510 hobbyist measurements.