Reef Tank Lighting Guide: PAR, Spectrum & LED Setup

1. Why Lighting Matters

Light is the single most important environmental factor for coral health and coloration. The vast majority of reef-building corals depend on symbiotic algae called zooxanthellae (genus Symbiodinium) that live within their tissue. These microscopic algae perform photosynthesis, converting light energy into sugars that supply up to 90% of a coral’s energy needs [1]. Without adequate light of the correct spectrum and intensity, zooxanthellae cannot photosynthesize effectively, and corals slowly starve [5].

This is why understanding PAR and PUR is essential for every reef keeper. PAR (Photosynthetically Active Radiation) measures the total amount of light in the 400–700 nm wavelength range that is available for photosynthesis, expressed in µmol/m²/s [5] [6]. However, not all wavelengths within that range are equally useful to corals.

PUR (Photosynthetically Usable Radiation) is the subset of PAR that zooxanthellae can actually absorb and use [11]. A light with high PAR but a spectrum dominated by green and yellow wavelengths may deliver less usable energy than a lower-PAR light concentrated in the blue and red peaks that zooxanthellae prefer [11]. In practice, this means PUR matters more than PAR for coral growth and coloration [4].

2. PAR Values by Coral Type

Different corals have evolved at different depths on the reef, and their light requirements reflect this. Shallow-water SPS corals need intense light, while deep-water soft corals thrive under moderate illumination [3] [5]. The table below shows recommended PAR ranges measured at the coral’s surface.

Species-Specific PAR Guidelines

3. Understanding Spectrum

Not all light is created equal when it comes to coral photosynthesis and coloration. The spectrum — the specific wavelengths of light emitted — determines whether your corals grow, color up, or bleach [4].

Key Wavelength Ranges

The blue wavelength range (420–470 nm) is the undisputed workhorse of reef lighting. It is the primary absorption peak for both chlorophyll a and the peridinin pigments in zooxanthellae [4]. This is why reef tanks look “blue” — the spectrum is optimized for what corals actually need, not for what looks natural to the human eye [9].

Blue light drives both growth and coloration. Corals produce fluorescent proteins as a natural “sunscreen” to protect against intense blue/UV light, and these fluorescent proteins are what create the vivid greens, oranges, pinks, and reds prized by reefers [4] [7].

White light promotes algae growth at high levels. Broad-spectrum white light in the 500–600 nm range is efficiently used by nuisance algae but less so by coral zooxanthellae [9]. Running white channels too high can shift the competitive advantage from corals to algae. BRS recommends keeping white/daylight channels at 30–50% of blue intensity [10].

4. LED Spectrum Settings

Modern LED fixtures offer multi-channel control, letting you dial in the exact spectrum for your tank. Below are popular spectrum presets used by experienced reefers, compiled from BRS [10], Current USA [7], and community settings [9].

5. Photoperiod Schedule

Corals on natural reefs experience a gradual sunrise, peak intensity at midday, and gradual sunset. Mimicking this cycle reduces stress and promotes natural behavior [8]. A well-designed photoperiod includes a ramp-up period, a peak period, and a ramp-down period [1].

Recommended Parameters

• Total photoperiod: 9–12 hours [8]
• Peak intensity: 6–8 hours [1] [8]
• Ramp up/down: 30–60 minutes each [8] [13]
• Moonlight (optional): 1–5% blue for 2–3 hours after sunset [7]

Sample Schedule

6. LED Product Comparison

The reef LED market ranges from budget-friendly units to professional-grade fixtures. Below are popular options organized by price tier, with PAR values measured at 12” depth in open air (actual in-tank values vary with mounting height and water depth) [2] [17].

Budget (< $100)

Mid-Range ($100–$300)

High-End ($300+)

7. New Coral Acclimation

New corals need time to adjust to your lighting conditions. Corals shipped from a vendor may have been under different spectrum, intensity, and photoperiod settings for months or years [13]. A sudden change from low light to high light is one of the most common causes of new coral bleaching and death [14].

Four Acclimation Methods

Acclimation Timeline

The full acclimation process should take 2–4 weeks minimum [13] [14]. SPS corals, particularly Acropora, may need up to 6 weeks if moving from low-light vendor conditions to a high-PAR display tank [15].

Signs of Light Stress

• Polyp retraction: Corals close up and stop extending polyps during the day [14]
• Tissue lightening: Gradual loss of color from the tips down (zooxanthellae expulsion) [16]
• Mucus production: Excessive slime coating as a stress response [15]
• Tissue recession: Skeleton becoming visible at the base or tips [16]
• Curling or shrinking: Soft corals and LPS pulling in or curling inward [14]

8. Too Much Light: Coral Bleaching

Coral bleaching occurs when corals expel their zooxanthellae in response to stress, leaving the translucent tissue over the white calcium carbonate skeleton [16]. While temperature stress is the primary cause of bleaching on natural reefs, in aquariums excessive lighting is one of the most common triggers [1] [16].

Symptoms of Light-Induced Bleaching

• Even whitening from the top down: Light bleaching typically starts at the highest-light areas (tips, upper surfaces) and progresses downward [16]
• Polyp retraction: Persistent daytime closure, especially near the light source [14]
• Color shift to pale or pastel: Before full bleaching, corals often go through a “pale” phase where color fades uniformly [16]
• Tissue remains intact: Unlike bacterial infections, light bleaching typically leaves tissue intact but transparent [16]

Distinguishing Light Bleaching from Disease

Contributing Factors

• New or upgraded light fixture without acclimation period [13] [14]
• Clear water after carbon or GFO change: Reduced tannins increase light penetration [16]
• Water level drop: Evaporation or maintenance reduces the water column, increasing PAR at coral surface [5]
• Cleaned glass: Removing algae film from the glass can dramatically increase light transmission [16]
• Combined heat + light stress: High temperatures lower corals’ ability to tolerate intense light [16]

9. PAR Meters

You cannot optimize what you do not measure. A PAR meter is the only way to know the actual light intensity at each point in your tank [5] [6]. Guessing based on fixture specs or mounting height is unreliable because PAR varies dramatically with depth, water clarity, and fixture age [2].

10. Frequently Asked Questions

References

Every factual claim in this guide is cited to its original source. Click any [n] in the text above to jump here.

1. Top Shelf Aquatics — “SPS Coral Lighting: PAR, Spectrum, and Photoperiod Explained”
2. Portland Aquarium — “LED Light for Reef Tank PAR Comparison”
3. Reefco Aquariums — “PAR Levels for Corals”
4. Reefco Aquariums — “Best Light Spectrum for Coral Growth”
5. Pod Your Reef — “Understanding PAR: A Hobbyist’s Guide to Reef Aquarium Lighting”
6. ATI North America — “What is PAR and How Much Do I Need?”
7. Current USA — “5 Popular Color Spectrums for Saltwater Aquariums”
8. The Beginners Reef — “How Long Should Reef Tank Lights Be On?”
9. OCD Reefs — “Blue vs White Light for Saltwater Aquariums”
10. BRS — “SPS Lighting Made Simple & Stable: The BRS/WWC System”
11. BRS — “PUR & Quality of Light Investigation”
12. BRS — “Red Sea ReefLED Investigation: PAR, Spectrum, and Everything In Between”
13. LiveAquaria — “Acclimating Corals to LED Lighting”
14. Current USA — “How to Properly Acclimate Your Corals to LED Lighting”
15. Orphek — “How to Acclimate Your Corals to LED Lighting”
16. BRS — “Coral Health Issues: Identification & Solutions”
17. Aquarium Specialty — “Understanding Reef Tank Lighting: High-End Options, Affordable Alternatives, and Why PAR Values Matter”