Reef Lighting Guide
Light is not optional in a reef tank — it is the energy source that powers the entire system. Most corals commonly kept in reef aquariums are photosynthetic, depending on symbiotic algae called zooxanthellae that live within coral tissue for the majority of their nutrition. Without adequate light of the right spectrum and intensity, corals cannot sustain themselves regardless of how well every other parameter is managed. Understanding how reef lighting works is essential for keeping corals alive and thriving.
This guide covers the fundamentals of reef aquarium lighting: PAR, spectrum, photoperiod, coral light requirements by category, and how to acclimate corals to a new fixture.
How Reef Lighting Works
Zooxanthellae and Photosynthesis
Most corals kept in reef aquariums host zooxanthellae — single-celled dinoflagellate algae — within their tissue. These algae photosynthesize using light, producing sugars and oxygen that the coral uses for energy and growth. In return, the coral provides the algae with shelter and nutrients. This symbiotic relationship is the foundation of coral reef ecosystems and the reason reef tanks require specialized lighting.
When corals are stressed by incorrect light levels, temperature spikes, or parameter instability, they expel their zooxanthellae — a process called bleaching. Bleached corals lose their color and their primary energy source. They can recover if conditions improve, but prolonged bleaching leads to death.
PAR: Measuring Light Intensity
PAR (Photosynthetically Active Radiation) measures the intensity of light in the wavelengths that drive photosynthesis (400–700nm). It is measured in μmol photons/m²/s, commonly called “micromoles” or simply “PAR.”
PAR is the most useful metric for reef lighting because it directly measures what corals use. Wattage, lumen output, and color temperature are not reliable indicators of whether a light will support coral growth — PAR is.
PAR decreases with depth and distance from the light source. A fixture that produces 300 PAR at 6 inches may produce 100 PAR at 18 inches. Always evaluate PAR at the depth where corals will actually be placed, not at the water surface. Additionally, two corals placed just six inches apart can receive very different PAR depending on rockwork, shadows, and fixture spread — measuring at each coral’s actual position is the only reliable way to know what it’s receiving.
Spectrum: The Color of Reef Light
Spectrum refers to the wavelengths of light a fixture produces. Reef corals and their zooxanthellae respond most strongly to blue and violet wavelengths (420–450nm), which penetrate water effectively and drive zooxanthellae photosynthesis. This is why reef tanks appear blue — the blue-heavy spectrum is functional, not just aesthetic.
A good reef light provides a full spectrum including:
- Violet (400–420nm): Drives coral fluorescence and zooxanthellae photosynthesis.
- Blue (420–490nm): The primary photosynthetic driver for reef corals. Penetrates water better than other wavelengths.
- Green (490–560nm): Green wavelengths contribute to the overall spectrum and penetrate water effectively, though blue and violet remain the primary wavelengths used by zooxanthellae.
- Red (620–700nm): Less important for reef corals than for freshwater plants, but contributes to full-spectrum output.
- White (broad spectrum): Provides the natural appearance of the tank and contributes to overall PAR.
PUR: Photosynthetically Usable Radiation
PUR is a refinement of PAR that accounts for the fact that not all wavelengths within the PAR range are equally useful to corals. A light that produces high PAR in wavelengths corals don’t use efficiently will be less effective than a light with lower total PAR but better spectral alignment with coral photosynthesis. This is why spectrum quality matters alongside raw PAR numbers.
Moonlights
Moonlights are primarily aesthetic. While they can provide enjoyable nighttime viewing and allow observation of nocturnal fish and invertebrate behavior, corals do not require moonlights to thrive. If used, keep them dim and avoid leaving them on all night — corals and fish benefit from a true dark period.
Coral Light Requirements by Category
Fish-Only with Live Rock (FOWLR)
No coral photosynthesis requirements. Any light that looks good and runs on a consistent schedule is adequate. PAR is not a meaningful consideration for fish-only systems.
Soft Corals
Leathers, mushrooms, zoanthids, xenia, Kenya tree
Target PAR: 50–150 μmol/m²/s
Soft corals are the most forgiving coral category for lighting. They adapt to a wide range of intensities and are the best starting point for beginner reef keepers. Most soft corals prefer indirect or moderate flow alongside moderate light.
LPS Corals
Hammer, torch, frogspawn, brain corals, elegance, bubble coral
Target PAR: 75–200 μmol/m²/s (species dependent)
LPS corals vary more in their light requirements than soft corals. Hammer and torch corals are relatively adaptable; elegance and brain corals can be more sensitive. Most LPS prefer moderate, indirect light rather than intense direct illumination.
SPS Corals
Acropora, montipora, stylophora, pocillopora
Target PAR: 200–350 μmol/m²/s (some species and mature systems run higher)
SPS corals are the most demanding coral category. They require high, stable light alongside high flow and very stable water chemistry. SPS systems are best suited to experienced reef keepers with mature, stable tanks. Acclimation is critical — even SPS corals that ultimately prefer high light can bleach if exposed to high PAR too quickly.
Photoperiod: How Long to Run Reef Lights
Most reef tanks run lights for 8–10 hours per day. A consistent photoperiod is more important than the exact duration — corals and fish benefit from predictable light and dark cycles. Use a timer or programmable controller to automate the schedule.
Running lights longer than 10–12 hours per day does not meaningfully benefit corals and increases the risk of nuisance algae growth. Shorter photoperiods (6–8 hours) can work for some systems but may limit coral growth rates.
Sunrise and sunset simulation — gradually ramping light intensity up and down at the start and end of the photoperiod — reduces stress on fish and corals and is available on most programmable reef fixtures.
Light Types for Reef Tanks
LED
LED is the dominant technology for reef lighting. Modern reef LEDs offer full spectrum control, programmable intensity and color channels, low heat output, and long fixture lifespan. The best reef LEDs (AI Hydra, EcoTech Radion, Kessil) produce PAR output competitive with T5 and metal halide at significantly lower operating cost. LED is the recommended choice for most new reef builds.
T5 Fluorescent
T5 fluorescent fixtures produce even, consistent PAR distribution across the entire tank footprint — a characteristic that LED fixtures can struggle to match without careful positioning. T5 remains popular among experienced SPS keepers for this reason. The tradeoff is ongoing bulb replacement cost (every 6–12 months) and higher heat output than LED. T5/LED hybrid fixtures combine the even spread of T5 with the spectrum flexibility of LED.
Metal Halide
Metal halide was the standard reef lighting technology before LED matured. It produces intense, natural-looking light with excellent PAR output and the shimmering “caustic” effect on the tank floor that many reef keepers find aesthetically appealing. Significant heat output, high electricity consumption, and bulb replacement costs make it less practical than LED for most modern reef setups. Still used in some large, established systems.
How to Acclimate Corals to a New Light
Coral bleaching from light shock is one of the most common mistakes when setting up a new reef fixture or adding new corals. Even corals that ultimately prefer high PAR can bleach if exposed to high intensity too quickly.
- Start new corals at 30–50% of your target intensity.
- Increase intensity by 10% every 1–2 weeks.
- Place new corals lower in the tank initially and move them up gradually as they acclimate.
- Watch for signs of stress: polyp retraction, pale color, or tissue recession. If these appear, reduce intensity and acclimate more slowly.
- When installing a new fixture over an established reef, reduce intensity to 50–60% initially even for corals that were previously thriving — different fixtures produce different spectra and the corals need time to adjust.
- Many modern LED fixtures include a built-in acclimation mode that automatically ramps intensity over several weeks — check your fixture’s app or settings before manually managing the ramp-up.
Reef Lighting FAQs
How do I know if my corals are getting enough light?
The most reliable method is a PAR meter — measure PAR at the location of each coral and compare to the target range for that coral category. Visually, corals receiving adequate light typically show extended polyps, good color, and visible growth over time. Corals receiving too little light may pale, retract polyps, or move toward the light source (in the case of mobile species like mushrooms).
What is coral bleaching and can corals recover?
Bleaching occurs when corals expel their zooxanthellae in response to stress — most commonly from too much light, too little light, temperature spikes, or parameter instability. Bleached corals turn white or pale and lose their primary energy source. They can recover if the stressor is removed promptly and conditions stabilize, but prolonged bleaching leads to starvation and death. Bleaching is one of the most important warning signs in a reef tank.
Do I need a PAR meter?
Not essential for beginners, but highly useful. PAR meters let you verify actual light intensity at coral placement rather than relying on manufacturer specifications. Many local reef clubs and fish stores rent PAR meters — worth using when setting up a new fixture or troubleshooting coral health issues.
Can I use a freshwater planted tank light on a reef?
Not recommended for coral tanks. Freshwater planted lights are optimized for green plant photosynthesis and typically lack the blue and violet wavelengths (420–450nm) that reef corals require. They may also lack sufficient PAR output for most coral species. For FOWLR tanks with no corals, a freshwater light can work adequately.
How long do reef LED fixtures last?
Quality reef LED fixtures typically last 5–10 years before LEDs begin to degrade meaningfully. Unlike T5 bulbs, LEDs don’t require scheduled replacement — but PAR output does decline gradually over time. If coral growth slows in an established tank with no other obvious cause, measuring PAR with a meter can reveal whether the fixture has degraded.