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Reef Lighting Guide: How to Choose and Set Up

Quick Summary

Choosing a reef light feels overwhelming because the options are vast and the terminology is dense. But the core decision is simpler than it appears: match the light to your corals and your tank dimensions. Soft corals and LPS need moderate light (75 to 200 PAR). SPS corals need strong light (250 to 450 PAR). The fixture needs to cover your tank's footprint with a spectrum heavy in blue and violet. Once you select the right fixture, setting it up correctly (proper height, intensity ramp, photoperiod) determines whether your corals thrive or struggle. This guide walks through the practical decisions from fixture selection to daily operation.

What Your Corals Actually Need from Light

Before comparing fixtures, it helps to understand what light does biologically. Most reef corals host symbiotic algae (zooxanthellae) in their tissue. These algae photosynthesize using light energy, producing sugars that the coral uses for growth, calcification, and tissue maintenance. In most stony corals, zooxanthellae provide 70 to 90% of the coral's total energy.

The light characteristics that matter are intensity (how much light reaches the coral), spectrum (which wavelengths the light contains), and duration (how many hours per day the light runs). Getting all three right creates the conditions for healthy, colorful corals. Getting any one wrong shows up as bleaching, browning, slow growth, or algae problems.

You will often notice that corals respond to lighting changes within days to weeks. A coral moved to brighter light extends fewer polyps and may pale slightly as it adjusts. A coral moved to dimmer light extends more polyps and may darken as zooxanthellae multiply to capture more energy. These responses are normal adaptation, not damage, as long as the change is gradual.

How to Choose a Reef Light Fixture

The fixture you need depends on three factors: what corals you plan to keep, how deep your tank is, and how wide it is.

Match the Light to Your Corals

Not all reef tanks need the same light. A tank full of mushrooms, zoanthids, and soft corals has very different lighting needs than an SPS-dominant system.

Coral Type PAR Needed Fixture Requirement
Soft corals, mushrooms 50 to 150 PAR Mid-range LED or basic T5
LPS corals (euphyllia, acans) 75 to 200 PAR Mid-range to high-quality LED
Mixed reef (softies + LPS + some SPS) 100 to 300 PAR Quality LED with adjustable intensity
SPS-dominant (acropora, montipora) 250 to 450 PAR High-output LED or hybrid LED/T5

If you are starting a reef tank and not sure what corals you will keep, choose a fixture that can deliver at least 200 to 300 PAR at the top of the rockwork. This covers mixed reef territory and gives you room to grow into SPS if you decide to later.

Account for Tank Depth

Water absorbs light. Every inch of depth reduces the PAR that reaches your corals. A fixture that delivers 300 PAR at 6 inches of water depth may only deliver 100 PAR at 24 inches.

Shallow tanks (under 18 inches) are the easiest to light. Almost any quality reef LED can deliver adequate PAR throughout. Deep tanks (24 inches or more) require higher-output fixtures or multiple fixtures to push sufficient light to the substrate.

If your tank is deeper than 20 inches and you plan to keep light-demanding corals at multiple depths, factor this into your fixture choice. Either choose a high-output fixture or plan to keep SPS only in the upper half of the tank where PAR is naturally stronger.

Account for Tank Width

A single LED fixture creates a cone of light that covers a limited footprint. Most puck-style LEDs (Kessil, AI Prime) cover approximately 24 by 24 inches effectively. Bar-style fixtures (Radion, AI Hydra, Orphek) cover wider areas but still have edges where PAR drops off.

For tanks wider than 24 inches, plan for multiple fixtures or a fixture specifically designed for wider coverage. Two fixtures mounted with overlapping coverage zones produce more even PAR across the width than a single centered fixture.

Popular Fixture Categories

Puck-Style LEDs

Compact, focused fixtures that mount above the tank on a gooseneck arm or hanging kit. They deliver high PAR in a focused area and work best for small to medium tanks (under 36 inches long).

Best for: Nano reefs, 20 to 30 gallon tanks, frag tanks.

Examples: Kessil A360X, AI Prime 16HD.

Pros: Compact footprint, strong shimmer effect, easy to mount. The Kessil series produces a natural shimmer effect that many reef keepers find visually appealing.

Cons: Limited coverage area. Larger tanks need two or more units. PAR drops off significantly at the edges.

Bar-Style LEDs

Rectangular fixtures that span part or all of the tank length. They typically offer channel-by-channel spectrum control and programmable schedules. This is the most common fixture type for tanks 36 inches and longer.

Best for: Medium to large tanks (36 to 72 inches).

Examples: Ecotech Radion XR15/XR30, AI Hydra 32/64, Orphek Atlantik, Red Sea ReefLED.

Pros: Wide coverage, advanced control, strong PAR output across the fixture footprint.

Cons: Higher cost. Multiple units needed for tanks over 48 inches. Some fixtures create less natural shimmer than puck-style lights.

T5 Fluorescent Fixtures

Multi-bulb fixtures using high-output fluorescent tubes. T5 fixtures produce extremely even, diffused light with no hot spots or shadows.

Best for: Any size tank, particularly those prioritizing even PAR distribution.

Examples: ATI Sunpower, Aquatic Life hybrid fixtures.

Pros: Even light distribution across the entire tank. Proven coral growth. Bulb selection allows spectrum customization.

Cons: Higher energy consumption than LEDs. Bulbs degrade and need replacement every 9 to 12 months. No programmable intensity control (unless paired with dimmable ballasts).

Hybrid Fixtures (LED + T5)

Combine LED fixtures with supplemental T5 tubes. The LEDs provide intensity and controllable spectrum while the T5 tubes fill in shadows and even out coverage.

Best for: Advanced reef keepers who want the benefits of both technologies. SPS-dominant large tanks.

Pros: Best of both worlds: intensity from LEDs, coverage from T5. Eliminates the shadowing and hot spot problems of LED-only setups.

Cons: Highest cost and highest energy consumption. More complex to set up and maintain.

For an in-depth technology comparison, see the LED vs T5 guide.

How to Mount Your Light

Fixture height above the water surface significantly affects PAR distribution. Mounting too low concentrates light in a small area with harsh hot spots. Mounting too high spreads light widely but reduces peak PAR.

General Height Guidelines

  • Puck-style LEDs: 6 to 10 inches above the water surface. Lower for nano tanks, higher for broader spread.
  • Bar-style LEDs: 8 to 12 inches above the water surface. Most manufacturers provide recommended mounting heights.
  • T5 fixtures: Typically rest on tank rails or legs, 2 to 4 inches above the water.

Start at the manufacturer's recommended height. If corals at the top of the rockwork show signs of too much light (bleaching, excessive mucus), raise the fixture 1 to 2 inches. If corals seem light-starved (browning, stretching toward the light), lower it.

Mounting Options

Gooseneck arms attach to the tank rim or stand and hold puck-style fixtures. They are adjustable and clean-looking.

Tank-mounted brackets sit on the tank rim and hold bar fixtures at a fixed height.

Hanging kits suspend fixtures from the ceiling or a canopy on cables. They allow easy height adjustment and are the most flexible option for larger setups.

Tank legs/rails support T5 fixtures directly on the tank rim. Simple and stable but not height-adjustable.

Setting Up Intensity

Most modern LED fixtures allow intensity control from 0 to 100%. Running your fixture at 100% from day one is one of the most common mistakes new reef keepers make. It overwhelms corals that were grown under different lighting at the store or frag farm.

Initial Setup

Start at 30 to 50% of your target intensity. This applies to both new tanks and existing tanks receiving a new fixture. Corals need time to acclimate to any change in light.

Ramp-Up Schedule

Increase intensity by approximately 10% every 3 to 5 days. Monitor corals during each increase for signs of stress (bleaching at the tips, tissue pulling back from the skeleton, excessive mucus production). If stress appears, reduce intensity by 10% and hold for a week before resuming.

A typical ramp-up from 40% to 80% takes three to four weeks. This gradual approach prevents photodamage and allows zooxanthellae to adjust their density to the new light level.

Finding Your Target Intensity

The correct intensity depends on your corals and your fixture's efficiency. A fixture running at 60% may produce more PAR than a different fixture at 100%. The only reliable way to know your actual PAR values is to measure with a PAR meter.

If you do not have a PAR meter, use coral behavior as your guide. Healthy corals under appropriate light show vibrant coloration, good polyp extension, and steady growth. Corals under too much light pale from the tips down. Corals under too little light turn brown and extend polyps excessively, trying to capture more energy.

For specific PAR targets by coral type, see the PAR for reef tank guide.

Setting Up Your Spectrum

Spectrum control is one of the main advantages of LED fixtures. Most reef LEDs offer individual channels for different wavelength ranges: UV, violet, royal blue, blue, white, red, and sometimes green or cyan.

A Practical Starting Point

For most reef tanks, this spectrum mix produces healthy corals and appealing aesthetics:

  • Royal blue (440 to 460 nm): 80 to 100%. This is the primary photosynthesis driver. Keep it high.
  • Blue (460 to 490 nm): 70 to 90%. Supplements royal blue and contributes to the reef aesthetic.
  • Violet/UV (380 to 420 nm): 40 to 70%. Stimulates fluorescent pigments in coral tissue. Enhances color pop.
  • White: 20 to 40%. Improves visual clarity and color rendering. Too much white washes out fluorescence and can fuel algae.
  • Red (620 to 700 nm): 5 to 15%. Small amounts improve color rendering. High red promotes algae without benefiting corals.
  • Green/Cyan (490 to 550 nm): 5 to 20%. Adds visual depth. Minimal biological impact.

The Blue vs. White Balance

In practice, the most impactful spectrum decision is the ratio of blue to white. More blue produces stronger coral fluorescence, a deeper reef aesthetic, and better matches the spectrum corals evolved under. More white makes the tank look brighter and more natural to human eyes but reduces fluorescent pop.

Most reef keepers find their preferred balance somewhere around 70 to 80% blue-weighted. Start here and adjust to taste. If your corals are coloring up nicely and growing well, your spectrum is working regardless of what the percentages read.

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

Setting Up Your Photoperiod

Photoperiod is how long your lights run each day. In most reef tanks, 8 to 10 hours of full-intensity lighting produces the best results.

A Standard Schedule

Here is a practical lighting schedule for a reef tank:

Time Phase Purpose
9:00 AM Blue-only dawn 30 to 60 minutes of low-intensity actinic light
9:30 AM Ramp up Gradual increase to full intensity over 30 to 60 minutes
10:00 AM Full intensity Primary photosynthesis period begins
8:00 PM Ramp down Gradual decrease over 30 to 60 minutes
8:30 PM Blue-only dusk 30 to 60 minutes of actinic viewing light
9:00 PM Moonlight or off Optional low-intensity blue overnight

Adjust the start time to match your viewing schedule. If you prefer to watch the tank in the evening, shift the entire schedule later. The total duration and ramp structure matter more than the specific clock times.

Why Not Run Lights Longer

Running lights for 12 or 14 hours does not help corals. Zooxanthellae reach photosynthetic saturation within 8 to 10 hours. Beyond that, additional light hours primarily benefit algae, which continue photosynthesizing efficiently after coral zooxanthellae have saturated. If you are battling algae, shortening the photoperiod to 6 to 7 hours temporarily (while addressing nutrients) can help shift the balance.

For detailed photoperiod guidance, see the how long to run reef lights guide.

Common Setup Mistakes

Starting at Full Intensity

New reef keepers often install a fixture and crank it to 100% on day one. Corals that were growing under moderate light at the store are suddenly blasted with twice the PAR they are adapted to. The result is bleaching within days. Always start low and ramp up over weeks.

Ignoring Coverage Gaps

A single fixture centered over a wide tank creates a bright center and dark edges. Corals placed at the edges may struggle while corals in the center are overlit. Use multiple fixtures or supplement with additional light for full coverage.

Mounting Too Low

Mounting a powerful LED 4 inches above the water creates an extremely bright hot spot directly below and dark areas everywhere else. The PAR gradient becomes too steep for diverse coral placement. Raise the fixture to spread light more evenly.

Too Much White Light

Turning white channels to 80 or 100% makes the tank bright to human eyes but provides minimal photosynthetic benefit compared to blue. It also promotes algae growth by providing wavelengths that algae use efficiently but that corals do not need in excess.

Skipping Acclimation After Bulb Changes

Replacing all T5 bulbs at once introduces significantly more PAR than the degraded bulbs they replaced. Replace one or two bulbs at a time, with two to three weeks between changes.

Not Measuring PAR

Relying on perceived brightness or fixture percentage without knowing actual PAR values leads to guesswork. Human eyes are most sensitive to green wavelengths, so a visually bright white light may deliver less useful PAR to corals than a dimmer-looking blue light. A PAR meter removes the guesswork entirely.

System Interactions

Water Clarity

Dissolved organics, tannins, and particulate matter reduce light penetration. Running activated carbon, maintaining a clean protein skimmer, and performing regular water changes keeps water crystal clear. If you have recently upgraded your light but corals still seem underwhelmed, test your water clarity before adding more intensity.

Nutrients and Color

Light intensity interacts directly with nutrient levels (nitrate and phosphate) to determine coral coloration. High light with low nutrients produces the most vivid fluorescent colors. High light with high nutrients causes browning as zooxanthellae overgrow. The best results come from appropriate light paired with controlled, low-level nutrients. See the nitrate guide and phosphate guide.

Flow

Every coral that receives strong light also needs adequate flow. Light drives photosynthesis, and flow delivers CO₂ and removes photosynthetic byproducts (including excess oxygen). Corals in stagnant water under intense light can develop localized tissue damage from oxygen toxicity. Ensure flow reaches every part of the illuminated rockwork.

Algae

Light fuels algae just as it fuels corals. The difference is that nutrient control determines which organism benefits most. With low nutrients, corals use the light energy for growth. With high nutrients, algae outcompetes corals for the available energy. Controlling nutrients before increasing light intensity is always the correct order.

Advanced: Using a PAR Meter

A PAR meter is the single most useful tool for optimizing reef lighting. It measures actual photon delivery at any point in your tank, removing all guesswork about intensity.

How to Map Your Tank

Take PAR readings at a grid of points across your tank:

  • Under the fixture center at the surface, mid-depth, and substrate
  • At the edges and corners at the same three depths
  • At key coral placement locations

Record the values and create a mental (or physical) map of your tank's light zones. Use this map to place corals according to their PAR requirements. SPS go where PAR is 250 to 450. LPS go where it is 75 to 200. Mushrooms and softies go in the 30 to 150 zone.

When to Remeasure

Remeasure PAR after any change to fixture height, intensity settings, or fixture count. Also remeasure if you rearrange rockwork, since rock structure creates shadows that affect PAR distribution below the light line.

Common Myths

"You need the most expensive light for a reef tank." Many mid-range LED fixtures grow beautiful corals. The price difference between a $200 fixture and a $600 fixture often comes down to build quality, spectrum control granularity, and brand ecosystem. If your fixture delivers appropriate PAR in the right spectrum, your corals will thrive regardless of price.

"Blue light is just cosmetic." Blue light (440 to 480 nm) is the single most important wavelength range for coral photosynthesis. It is biologically essential, not merely aesthetic.

"More hours of light means more coral growth." Photosynthesis saturates within 8 to 10 hours. Additional light hours primarily benefit algae. Extended photoperiods are one of the most common causes of algae blooms in new reef tanks.

"Corals need natural sunlight." Artificial reef lights are designed to replicate the specific wavelengths corals use. Direct sunlight through a window provides uncontrolled intensity, excessive heat, and wavelengths that promote algae. Artificial lighting is superior for reef tank purposes.

"PAR does not matter if corals look healthy." Visual assessment is useful but imprecise. Corals can appear fine while slowly declining from insufficient light, and by the time the problem becomes visible, tissue loss may already be significant. PAR measurement provides early, objective data.

FAQ

What is the best light for a beginner reef tank?

A quality bar-style LED with adjustable intensity and spectrum control. The AI Hydra, Ecotech Radion, or similar mid-to-high range fixture covers most beginner reef needs and can grow with your coral collection.

How much should I spend on a reef light?

For a 3 to 4 foot tank, expect to spend $200 to $500 for a quality LED fixture. This is an investment that directly determines whether your corals thrive. A cheap, insufficient light will cost more in the long run through failed corals and eventual replacement.

Can I use two different brand fixtures on one tank?

Yes, but matching spectrum and intensity between different brands requires some experimentation. If possible, use identical fixtures for simplicity. If mixing brands, use a PAR meter to verify even coverage.

How do I know if I need a second light?

If PAR at the edges of your tank is less than half the PAR at the center, a second fixture will improve coverage. You will often notice corals at the edges growing slower or losing color compared to those in the center.

Should I run moonlights overnight?

Moonlights (very low intensity blue LEDs) are optional and primarily aesthetic. They do not significantly affect coral biology at the intensity levels used. Some reef keepers enjoy the nighttime viewing they provide.

How long do LED fixtures last?

Quality LED fixtures last 5 to 10 years or more. Individual LEDs may degrade in output over time (typically 10 to 20% over 50,000 hours). If you notice declining coral growth after several years, the LEDs may need replacement or the fixture may need upgrading.

Do I need UV light for corals?

UV wavelengths (380 to 420 nm) stimulate fluorescent pigment production in corals and enhance color expression. Most quality reef LEDs include UV channels. Corals can survive without UV, but their coloration is typically less vibrant.

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