Reef Tank Lighting Guide: PAR, Spectrum, LED vs T5, and What Actually Matters

Of all the gear debates in reef keeping — skimmer brands, pump choices, flow patterns — nothing generates more argument and more spending than lighting. Hobbyists buy flagship LED fixtures, tune spectral channels obsessively, and still wonder why their corals are not growing or coloring up. The reason is usually not the fixture. It is PAR, stability, and a realistic photoperiod.

This guide covers the science behind reef lighting and the practical decisions you will need to make.

What PAR actually measures

PAR stands for Photosynthetically Active Radiation. It measures the quantity of photons in the wavelength range that drives photosynthesis (roughly 400–700 nm), expressed in micromoles of photons per square meter per second (µmol/m²/s). When a reef keeper says "my corals need 250 PAR," they mean they need 250 µmol/m²/s of usable light energy hitting that coral's surface.

PAR is not the same as watts, lumens, or the light intensity number in your LED app. Watts measure energy consumption, lumens measure perceived brightness by the human eye, and your fixture's "intensity" percentage is a relative setting, not a calibrated measurement. The only way to know your actual PAR is to measure it with an apogee quantum meter or a PAR meter (Seneye, Apogee MQ-510, or renting a meter from a local reef club).

PAR targets by coral type

PAR requirements vary significantly across the three coral categories, which is why fixture placement and intensity matter as much as fixture choice.

• Soft corals (softies): 50–100 µmol/m²/s. Mushrooms and leathers often prefer shade. Placing softies at the bottom of the tank or in lower-flow, lower-light areas is appropriate.
• LPS corals: 100–250 µmol/m²/s. Most Euphyllia (hammer, torch, frogspawn) and brain corals fall here. High-end LPS like Trachyphyllia can handle more, but most do well in this range.
• SPS corals: 200–400+ µmol/m²/s. Acropora in the upper third of a well-lit tank often sees 300–450 PAR. More is not always better — acropora can bleach under excessively high PAR if not acclimated.

In a mixed reef, this creates a natural depth gradient: softies on the sandbed and shaded overhangs, LPS in the middle, SPS at the top closest to the light. Plan rockwork with this in mind.

Why spectrum matters — and the blue-heavy look

Zooxanthellae absorb light most efficiently in the blue range — primarily 420–450 nm (blue/violet) and secondarily around 680 nm (red). Blue light penetrates water further than other wavelengths (which is why deep ocean looks blue, not green), and in a shallow aquarium it is the most efficient driver of photosynthesis per watt consumed.

This is why reef tanks look so blue. It is not aesthetic preference — it is the optimal spectrum for coral health. That said, pure blue lighting makes the tank look like a blacklight poster and makes it difficult to assess coral color and health. White and warm-white channels in LED fixtures improve visual contrast and bring out the natural colors of corals (especially reds, oranges, and yellows) without significantly reducing photosynthetic efficiency. The common practice is to run blue channels at 70–100% and white channels at 20–50%.

Some corals also produce fluorescent proteins that absorb specific wavelengths and re-emit them at a lower energy wavelength — this is why many corals glow dramatically under blue-heavy lighting and look almost brown under full-spectrum white light. The colors are real; you're just choosing which ones to reveal.

LED lighting

LEDs now dominate reef lighting for good reasons: programmable dawn-to-dusk ramps, full spectrum control across multiple channels, low heat output, low electricity consumption relative to output, and longevity (LEDs typically last 50,000+ hours before significant degradation). The ability to simulate cloud passing effects, lunar cycles, and weather storms is genuinely useful for some systems and mostly novelty for others.

Popular LED brands

• EcoTech Radion (XR15, XR30): The benchmark fixture. Excellent PAR distribution, deep spectrum programmability via the Mobius app, industry-leading support. Expensive ($600–$1,200 per fixture), but holds resale value well. The XR30 G6 Pro is widely considered the best single-fixture option for demanding SPS tanks under 36 inches wide.
• AI Hydra (26, 64): The value leader. Excellent PAR output, good spectrum, controllable via the myAI app. The Hydra 26 is sufficient for most LPS and soft coral tanks, and a pair of Hydra 64s covers a 4-foot SPS tank effectively at a fraction of the Radion price.
• Kessil A360X: Boutique quality with a signature shimmer effect. Uses a proprietary dense matrix LED rather than individual diodes, which produces crisp light patterns popular with SPS hobbyists. Limited channel control compared to Radion/Hydra but beloved by those who prefer simpler operation.
• Maxspect Recurve / Ethereal: Strong mid-range options with broad coverage patterns, popular in Europe. Good PAR for the price, decent app control.
• Reef Breeders Photon: Budget-to-mid-range LED with solid PAR output. Popular among hobbyists who want proven results without flagship pricing.

T5 lighting

T5 fluorescent tubes were the standard reef lighting technology before LEDs matured, and they still have genuine advantages that LED cannot fully replicate.

• Even spread: T5 tubes provide a flat, even plane of light with no hot spots and no shimmer. This is excellent for SPS frags placed across the tank at consistent depths and makes PAR prediction much more reliable.
• Proven SPS growth: Many of the most heavily grown SPS tanks in the world — especially competition tanks — ran T5 for decades. There is no question it grows coral.
• Color output: The combination of blue, actinic, and white T5 tubes produces a visually balanced look that many hobbyists prefer.

The downside: T5 tubes must be replaced every 9–12 months (they lose spectrum before they lose brightness, and old tubes can stall coral growth without any obvious visual cue), they run hot, and they consume significantly more electricity than LEDs for equivalent PAR.

The current best practice for many serious SPS hobbyists is a hybrid: LED fixtures for spectrum control and programmability, with a T5 supplement for even coverage and shimmer reduction. The ATI Powermodule (T5 rack with LED inserts) is a popular commercial hybrid solution.

Metal halide

Metal halides produce intense, natural-looking light with a beautiful shimmer. They were the dominant high-PAR solution before LEDs matured. Their weaknesses are significant: very high heat output requiring dedicated chilling systems, high electricity cost, and bulb replacement every 12–18 months. New builds almost never spec metal halide in 2026. Existing setups running halides are typically retained only because switching to LEDs requires recalibrating coral placement and would disrupt an established tank.

Photoperiod: how long to run the lights

Most reef tanks run lights for 8–10 hours per day. This is enough time for zooxanthellae to accumulate daily photosynthetic energy without driving excessive algae growth. Tanks with aggressive algae problems sometimes benefit from shorter photoperiods (6–7 hours) while the underlying nutrient issue is addressed.

Do not run a new tank at full intensity from day one. Corals must acclimate to new light levels the same way they acclimate to new water parameters. Start at 30–40% intensity and ramp up over 4–6 weeks. Sudden full-intensity exposure to a new fixture causes light shock — polyps retract, coral browns out, and bleaching can follow.

Dawn and dusk ramps (typically 30–60 minutes of ramping up in the morning and ramping down in the evening) are easy to program into any modern LED and help reduce stress on the system.

Some hobbyists run a "siesta" schedule — for example, lights on from 10am–2pm and 5pm–10pm, with lights off in the middle of the day. The claimed benefit is pH stabilization and reduced algae. Results are mixed, and it complicates viewing schedules. Most tanks do fine with a simple single continuous photoperiod.