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Reef Lighting Complete Guide: Everything You Need

Quick Summary

Lighting is one of the three pillars of a reef tank, alongside water chemistry and flow. Most reef corals depend on light for the majority of their energy through photosynthetic zooxanthellae. The right light provides the correct intensity (measured as PAR), the correct spectrum (heavy in blue and violet wavelengths), and runs for the right duration (8 to 10 hours daily). Getting lighting wrong shows up as bleaching, browning, slow growth, or algae outbreaks. Getting it right creates the conditions where corals color up, grow steadily, and thrive. This guide covers everything from the basics of how reef lighting works to advanced spectrum tuning and fixture selection.

Why Lighting Matters in a Reef Tank

Walk up to a reef tank with healthy corals and the first thing you notice is color. Vivid greens, deep purples, electric blues, fluorescent reds. That color is not decorative. It is biological, and it depends almost entirely on light.

Most reef corals are photosynthetic organisms. They host symbiotic algae called zooxanthellae within their tissue. These algae use light energy to photosynthesize, producing sugars and oxygen that the coral uses for energy, growth, and calcification. In most stony corals, zooxanthellae provide 70 to 90% of the coral's total energy budget. The remaining 10 to 30% comes from heterotrophic feeding (capturing particles from the water).

This means the light above your tank is not just illumination. It is the primary energy source for your corals. Too little light and corals cannot produce enough energy to grow or maintain their tissue. Too much light and zooxanthellae become overwhelmed, produce damaging reactive oxygen species, and the coral expels them in a bleaching response. The goal is finding the range where photosynthesis is efficient, the coral is energized, and the fluorescent pigments the coral produces for photoprotection create the vibrant coloration reef keepers seek.

Understanding PAR

PAR (Photosynthetically Active Radiation) is the standard measurement for light intensity relevant to photosynthetic organisms. It measures the number of photons in the 400 to 700 nanometer wavelength range that reach a given surface per second, expressed as micromoles per square meter per second (µmol/m²/s).

In practical terms, PAR tells you how much usable light is reaching your corals. A PAR reading of 200 at the substrate means corals placed there receive 200 µmol of photosynthetically active photons per second per square meter. A reading of 500 at the top of the rock structure means corals placed high receive significantly more light energy.

PAR varies dramatically within a single tank. The highest PAR is directly under the light source at the water surface. PAR decreases with depth (water absorbs light), with distance from center (light spreads outward), and in shaded areas behind rocks. A single reef tank can have PAR values ranging from 50 in shaded lower areas to 500 or more at the top of the rockwork directly under the fixture.

This gradient is actually useful. It allows you to place different corals at different PAR levels within the same tank. SPS corals that need high light go at the top. LPS corals that prefer moderate light go in the middle. Soft corals and mushrooms that tolerate low light go near the bottom or in shaded overhangs.

For detailed PAR targets by coral type, see the PAR for reef tank guide and coral PAR levels guide.

PAR Targets by Coral Type

Different corals evolved at different depths on natural reefs and have adapted to different light intensities. Matching PAR to coral type is one of the most important factors in reef lighting.

Here is a practical reference for PAR targets:

Coral Type PAR Range (µmol) Placement
SPS (Acropora, Montipora, Stylophora) 250 to 450+ Upper third of tank, directly under light
LPS (Euphyllia, Acanthastrea, Blastomussa) 75 to 200 Middle of tank, indirect light
Soft corals (leathers, sinularia) 50 to 150 Mid to lower tank
Mushrooms (discosoma, rhodactis) 30 to 100 Lower tank, shaded areas
Zoanthids and palythoa 75 to 250 Mid tank, adaptable
Clams (Tridacna) 200 to 400 Upper half, under direct light

These ranges are guidelines, not rigid requirements. Most corals can adapt to a range of PAR levels if the change is gradual. A coral moved from 100 PAR to 300 PAR overnight may bleach, while the same coral moved from 100 to 150 to 200 to 300 over several weeks will adapt successfully.

Light Spectrum for Reef Tanks

Spectrum describes the color composition of the light, specifically which wavelengths are present and in what proportion. In reef lighting, spectrum affects both coral biology and visual appearance.

The Blue Dominance

Natural sunlight on a shallow reef contains a full spectrum from ultraviolet through red. But water absorbs red and orange wavelengths rapidly. By the time light reaches 10 to 20 meters depth (where many reef corals live), the spectrum is dominated by blue and violet wavelengths (420 to 480 nm). This is why reef corals evolved to thrive under blue-heavy light.

For reef tanks, a spectrum emphasizing blue and violet (420 to 480 nm) with some UV (380 to 420 nm) and white supplementation produces the best combination of coral health and visual pop. Blue light drives photosynthesis efficiently in zooxanthellae and excites the fluorescent pigments in coral tissue that produce the vivid colors reef keepers value.

Key Wavelength Ranges

Most reef keepers benefit from understanding the major wavelength groups and what they do:

  • UV (380 to 400 nm): Stimulates fluorescent protein production in corals. Enhances color expression, particularly greens, oranges, and reds. Excessive UV can damage tissue.
  • Violet (400 to 420 nm): Strong driver of fluorescent pigment excitation. Produces the intense "pop" of fluorescent corals under actinic lighting.
  • Royal Blue (440 to 460 nm): The primary photosynthesis driver for zooxanthellae. The most important single wavelength range for coral health and growth.
  • Blue (460 to 490 nm): Supplements royal blue for photosynthesis and contributes to the overall blue reef aesthetic.
  • Cyan/Green (490 to 550 nm): Minimal photosynthetic value but adds visual depth and color rendering. Too much green makes corals look washed out.
  • Red (620 to 700 nm): Absorbed rapidly by water. Adds warmth and improves color rendering of reds and oranges at shallow depths. Excessive red promotes algae without benefiting corals at depth.
  • White (broad spectrum): Improves visual clarity and color rendering. A small amount of white mixed with blue creates a balanced reef appearance. Too much white shifts the tank toward a washed-out look and can fuel algae.

For a detailed breakdown of spectrum tuning, see the reef light spectrum guide.

Practical Spectrum Settings

Most modern LED fixtures allow channel-by-channel spectrum control. A good starting point for a reef tank:

  • Royal blue: 80 to 100%
  • Blue: 70 to 90%
  • Violet/UV: 50 to 70%
  • White: 20 to 40%
  • Red: 5 to 15%
  • Green: 5 to 15%

Adjust from there based on visual preference and coral response. If corals are browning, increase blue intensity or reduce white. If corals are bleaching, reduce overall intensity before adjusting spectrum.

LED vs. T5 Fluorescent

The two dominant lighting technologies for reef tanks are LED and T5 fluorescent. Both can grow any type of coral successfully. The choice comes down to control, coverage, cost, and personal preference.

LED Fixtures

LEDs are the most popular choice for modern reef tanks. They are energy-efficient, long-lasting, and offer precise control over intensity and spectrum through programmable controllers.

Advantages:

  • Adjustable intensity and spectrum (channel-by-channel control)
  • Programmable sunrise, sunset, and moonlight schedules
  • Lower electricity consumption per unit of PAR
  • Long lifespan (50,000+ hours for quality LEDs)
  • Compact form factor

Disadvantages:

  • Point-source lighting creates defined hot spots and shadows (the "disco ball" shimmer effect)
  • PAR distribution is less even than T5, requiring careful placement or multiple fixtures
  • Quality fixtures are expensive upfront
  • Cheaper LEDs may lack sufficient blue and UV wavelengths

Popular reef LED fixtures: Ecotech Radion, AI Hydra, Kessil, Orphek, Reef Factory, Red Sea ReefLED. The market is broad, and quality varies significantly. A good reef LED should provide strong output in the 400 to 480 nm range and offer channel-by-channel control.

T5 Fluorescent

T5 fixtures use high-output fluorescent tubes in a multi-bulb reflector housing. They were the standard reef lighting before LEDs became dominant and remain popular for their even light distribution.

Advantages:

  • Extremely even PAR distribution (no hot spots or shadows)
  • Soft, diffused light that blends naturally
  • Proven track record for growing all coral types
  • Tube selection allows spectrum customization

Disadvantages:

  • Higher electricity consumption than LEDs for equivalent PAR
  • Bulbs degrade and need replacement every 9 to 12 months
  • No intensity control (on or off only, unless using a dimming ballast)
  • Larger physical footprint
  • Heat output is higher

Hybrid (LED + T5)

Many advanced reef keepers run hybrid setups that combine LED fixtures with supplemental T5 tubes. The LEDs provide intensity, controllability, and spectrum customization, while the T5 tubes fill in shadows and even out PAR distribution. This approach gives the best of both technologies but at higher cost and complexity.

For a detailed comparison, see the LED vs T5 guide.

How Long to Run Reef Lights

Photoperiod (the number of hours lights are on per day) affects coral health, algae growth, and overall system stability.

Most reef tanks perform best with an 8 to 10 hour photoperiod at full intensity, with an additional 1 to 2 hours of ramp-up (sunrise) and ramp-down (sunset) on each end.

A typical lighting schedule looks like this:

Phase Duration Purpose
Actinic/blue only (dawn) 30 to 60 minutes Gradual wake-up, fluorescent display
Ramp to full intensity 30 to 60 minutes Gradual increase prevents light shock
Full intensity 8 to 10 hours Primary photosynthesis period
Ramp to blue only (dusk) 30 to 60 minutes Gradual reduction
Actinic/blue only (dusk) 30 to 60 minutes Fluorescent viewing period
Moonlight (optional) Overnight Low-intensity blue for nighttime viewing

Running lights longer than 10 hours at full intensity does not benefit corals. Zooxanthellae reach photosynthetic saturation within the 8 to 10 hour window. Extended photoperiods primarily fuel algae growth, since algae continue photosynthesizing efficiently beyond the point where coral zooxanthellae are saturated.

If you are dealing with algae issues, reducing the photoperiod to 6 to 7 hours temporarily can help tip the balance while you address nutrients. Once nutrients are controlled, return to the standard 8 to 10 hour schedule.

For a detailed guide on photoperiod optimization, see the how long to run reef lights guide.

Choosing the Right Light for Your Tank

Selecting a reef light depends on tank size, coral ambitions, and budget. Here is a practical framework for matching fixtures to reef tanks.

Small Tanks (Under 30 Gallons)

Nano reef tanks need compact fixtures with good PAR output and spectrum control. A single puck-style LED (like a Kessil A160 or AI Prime) typically covers a 24 by 24 inch footprint with adequate PAR for mixed reef corals.

Look for: adjustable intensity, blue-heavy spectrum, and a mounting option that fits your tank. Nano tanks are forgiving because the short water depth means even modest fixtures can deliver sufficient PAR to the substrate.

Medium Tanks (30 to 75 Gallons)

Most 3 to 4 foot tanks need either one large LED fixture or two smaller fixtures for even coverage. Uneven PAR distribution (high center, dark edges) is the most common lighting problem in this size range.

Two fixtures mounted 12 to 18 inches apart and slightly overlapping their coverage zones produces more even PAR than a single centered fixture. Measure PAR at multiple points across the tank to verify coverage before finalizing coral placement.

Large Tanks (75+ Gallons)

Larger tanks benefit from multiple LED fixtures, hybrid LED/T5 setups, or multi-tube T5 fixtures. The goal is even PAR distribution across the entire footprint. Three or more LED fixtures in a row, or a full-length T5 setup, provides the coverage these tanks require.

For SPS-heavy large tanks, PAR uniformity becomes critical. Dead zones with low PAR will not support SPS growth, and hot spots with extreme PAR can bleach corals. Use a PAR meter to map your tank after installation and adjust fixture height and intensity accordingly.

Budget Considerations

Quality reef lighting is an investment. Cheap LED fixtures often lack the spectrum, intensity, and longevity needed for a thriving reef. However, this does not mean the most expensive fixture is always the best choice.

For a soft coral or LPS tank, a mid-range LED fixture providing 100 to 200 PAR at the substrate is perfectly adequate. For an SPS tank, invest in a fixture that can deliver 300+ PAR at the rockwork peaks with good blue and violet output. The fixture should be a fixture you can grow into, not one you need to replace in a year.

Acclimating Corals to New Lighting

When installing a new light or moving corals to a brighter position, acclimation is essential. Corals adapt to their current light level, and a sudden increase causes photodamage that ranges from temporary stress to permanent bleaching.

For a new fixture: Start at 30 to 50% of your target intensity. Increase by 10% every three to five days over three to four weeks until you reach full power. Monitor corals for signs of stress (bleaching, tissue recession, excessive mucus) during the ramp-up. If stress appears, reduce intensity by 10% and hold for a week before resuming the increase.

For moving a coral to a brighter position: Place the coral in a shaded area first, then gradually move it higher (toward brighter light) over two to three weeks. Moving a coral from the sand bed to the top of the rockwork in a single step is one of the most common causes of bleaching in otherwise healthy tanks.

For replacing T5 bulbs: New T5 tubes are significantly brighter than degraded bulbs. Replace one or two tubes at a time (not all at once) and space replacements two to three weeks apart. This prevents a sudden PAR spike across the entire tank.

System Interactions

Water Clarity

Dirty water absorbs and scatters light, reducing PAR at depth. Activated carbon, regular water changes, and effective protein skimming keep water crystal clear, allowing maximum light penetration. If you have increased lighting intensity but corals still seem light-starved, check water clarity before adding more fixtures.

Nutrients and Coloration

Light intensity interacts with nutrient levels to determine coral coloration. High light with low nutrients produces the brightest fluorescent colors. High light with high nutrients causes browning (zooxanthellae overgrowth). Low light with low nutrients leads to paling. The ideal is adequate PAR combined with controlled, low-level nutrients. See the nitrate guide and phosphate guide.

Calcification

Light drives photosynthesis, which fuels calcification. Corals calcify faster during the day under strong light than at night. Adequate lighting is a prerequisite for healthy calcium and alkalinity consumption. If your tank consumes less calcium and alkalinity than expected for its coral load, insufficient light may be a contributing factor.

Algae

Light fuels both coral and algae photosynthesis. The difference is that corals are largely sessile and depend on their specific PAR zone, while algae colonize any illuminated surface. Controlling nutrients (nitrate and phosphate) determines whether available light energy grows coral or algae. Excessive photoperiod (beyond 10 hours) disproportionately benefits algae.

Flow

Light and flow interact at the coral surface. Adequate flow delivers CO₂ and nutrients to coral tissue and removes waste products from photosynthesis. Corals in stagnant water under strong light can develop localized oxygen toxicity from trapped photosynthetic byproducts. Ensure every coral that receives strong light also receives moderate to strong flow.

Advanced: PAR Measurement and Mapping

A PAR meter is the only way to know exactly how much light your corals receive. Perceived brightness to the human eye does not correlate reliably with PAR because our eyes are most sensitive to green wavelengths, while corals primarily use blue.

The Apogee MQ-510 and Seneye Reef are the most commonly used PAR meters in the reef hobby. The Apogee is considered the gold standard for accuracy. The Seneye is more affordable and provides continuous monitoring.

To map your tank, measure PAR at multiple points:

  • Surface center (directly under fixture)
  • Surface edges (corners and sides)
  • Mid-depth center
  • Mid-depth edges
  • Substrate center
  • Substrate edges

Record these values and use them to guide coral placement. Revisit the measurements after any fixture height change, intensity adjustment, or addition of a new fixture.

In most tanks, the PAR gradient from top to bottom spans a 3 to 5x range. A tank with 400 PAR at the surface center might have 80 to 120 PAR at the substrate edges. This is normal and actually beneficial for creating diverse coral habitats within a single system.

Advanced: Inverse Square Law and Fixture Height

The intensity of light decreases with the square of the distance from the source. This means small changes in fixture height have outsized effects on PAR.

Moving a fixture from 8 inches above the water to 12 inches above the water reduces PAR by approximately 44% (not 33% as a linear relationship would suggest). Conversely, lowering the fixture from 12 to 8 inches increases PAR by roughly 78%.

This relationship explains why fixture height is one of the most powerful tools for controlling PAR. If corals at the top of your rockwork are bleaching, raising the fixture by 2 to 3 inches can reduce PAR enough to resolve the issue without changing any electrical settings. If you need more PAR at depth, lowering the fixture is more effective than increasing intensity (which also increases heat and energy consumption).

The trade-off: lower fixtures concentrate PAR in a smaller footprint, creating sharper hot spots and darker edges. Higher fixtures spread light over a wider area but at lower peak intensity. Finding the optimal height for your tank requires balancing coverage and intensity, ideally guided by PAR meter measurements.

Common Myths

"More light is always better for corals." Corals have optimal PAR ranges. Exceeding those ranges causes bleaching, not faster growth. SPS corals thrive at 250 to 450 PAR. Pushing to 700 or 800 PAR does not accelerate growth and risks photodamage.

"White light causes algae." Algae grows in response to nutrients and photoperiod, not white light specifically. A well-balanced spectrum with white included does not cause algae if nutrients are controlled. However, excessive white relative to blue can promote algae while reducing coral fluorescence.

"You need the most expensive light for a reef tank." Many mid-range LED fixtures grow corals beautifully. The most expensive fixtures offer more control, wider coverage, and premium build quality, but they are not required for healthy coral growth. Match the fixture to your coral ambitions and tank size.

"Blue light is just for aesthetics." Blue light (440 to 480 nm) is the primary driver of photosynthesis in zooxanthellae. It is biologically essential, not just visually appealing. A reef run on all-white light would starve corals of their most efficient photosynthetic wavelengths.

"T5 is outdated technology." T5 fluorescent lighting remains one of the most effective reef lighting technologies. Its even PAR distribution, proven spectrum options, and reliability make it a valid choice. Many competitive coral growers and propagation facilities still use T5 or hybrid setups.

FAQ

What PAR do I need for a reef tank?

It depends on your coral types. SPS corals need 250 to 450 PAR. LPS corals thrive at 75 to 200 PAR. Soft corals and mushrooms do well at 30 to 150 PAR. A mixed reef needs a range of PAR levels, which the natural depth gradient in your tank provides.

How long should reef lights be on?

Run full-intensity lighting for 8 to 10 hours per day. Add 30 to 60 minutes of ramp-up and ramp-down on each end. Longer photoperiods benefit algae more than corals.

Are LED or T5 lights better for a reef tank?

Both grow corals effectively. LEDs offer more control, lower energy use, and longer lifespan. T5s provide more even light distribution. Hybrid setups combine the strengths of both. Choose based on your priorities and budget.

How do I know if my corals are getting enough light?

Signs of insufficient light: browning (zooxanthellae overgrowth to capture more light), stretching toward the light source, slow growth. Signs of too much light: bleaching (loss of color), tissue recession, excessive mucus. A PAR meter provides objective measurement.

Do I need a PAR meter?

A PAR meter is not strictly necessary but is highly recommended for SPS-dominant tanks. It removes guesswork from coral placement and fixture adjustment. If you cannot invest in a PAR meter, many local reef clubs loan them to members.

How high should I mount my LED fixture?

Start at 8 to 12 inches above the water surface. Lower for more intensity and a narrower spread. Higher for less intensity and a wider spread. Measure PAR at multiple points and adjust to achieve your target values across the tank.

Can I use freshwater lights for a reef tank?

Most freshwater planted tank lights lack the blue and violet wavelength output that reef corals need. Some high-end freshwater fixtures with strong blue channels can work for soft corals, but purpose-built reef lights are recommended for any tank with stony corals.

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