Short answer: choose the Radion XR15 G6 when a smaller light footprint, flexible multi-fixture placement, or precise coverage around braces and separated coral islands matters most. Choose the XR30 G6 when you want broader coverage from each fixture over a wide, open aquascape. Tank length, front-to-back width, rockwork, obstructions, coral placement, and the uniformity you expect are more useful than gallon capacity alone.

XR15 vs. XR30 at a glance

Both sizes use EcoTech Marine’s HEI2 optical system with a 126.3-degree spread. Both are programmable through the Mobius app on current iOS or Android devices without requiring a separate controller for basic operation. MXM integration is also available for control through Neptune Apex and Apex Fusion.

The size decision is separate from the spectrum decision. The XR15 is available as the blue-weighted Radion XR15 G6 Blue or balanced-spectrum Radion XR15 G6 Pro. The larger fixture is offered as the Radion XR30 G6 Blue and Radion XR30 G6 Pro.

Why tank dimensions matter more than gallons

Gallons describe water volume, but a reef light illuminates a physical footprint. Two aquariums can hold a similar volume while having very different lengths, widths, and heights. A long, narrow aquarium asks a light array to cover distance from end to end. A cube concentrates a similar volume beneath a shorter span. A wide peninsula or lagoon places more emphasis on front-to-back coverage.

Start with the tank’s actual display dimensions and compare them with EcoTech’s recommended footprints: 24 by 24 inches for the XR15 and 36 by 30 inches for the XR30. These recommended figures are better planning references than the maximum spreads of 30 by 30 inches and 48 by 30 inches. Maximum spread tells you how far light can extend, but it should not automatically be treated as a promise of identical illumination at every point.

Tank height still matters to the overall lighting plan, but the supplied specifications do not provide depth-specific output measurements or coral-specific PAR guarantees. That makes it important not to choose solely from a gallon chart or assume that reaching the glass means every coral position receives the same light. Rock height, coral level, mounting position, and shading all affect the practical result.

Length determines how many coverage zones you need

Visualize the display from above and divide its usable coral area into lighting zones. An XR15 is planned around a recommended 24-inch length, while an XR30 is planned around 36 inches. A tank longer than one recommended footprint may need additional fixtures, but the correct count depends on whether coral occupies the full length and how much overlap you want.

A sparse aquascape with coral concentrated in the middle does not necessarily need the same edge-to-edge emphasis as a reef wall filled from one end to the other. Conversely, using only the maximum spread to reach both ends can be a poor fit when uniformity is the priority. Overlapping recommended footprints gives more placement flexibility than stretching each fixture to its maximum published reach.

Front-to-back width can decide the matchup

The XR15’s recommended footprint is 24 inches front to back, compared with 30 inches for the XR30. That six-inch difference can matter on a wide display, especially when coral is placed near both the front and rear glass. If the reef structure occupies only a narrow central ridge, the smaller footprint may still align well with the actual coral zone. If rockwork and coral use most of a 30-inch-wide display, the XR30’s recommended footprint is the more direct match.

Measure the coral-bearing area rather than assuming every inch of sand must receive the same emphasis. Open sand, isolated rock, and negative-space aquascapes can change where useful coverage is needed.

How aquascape changes the answer

Fixture specifications describe an unobstructed footprint, but a reef is a three-dimensional structure. Tall rock pillars, shelves, branching colonies, and dense walls can shade lower or neighboring areas. EcoTech’s wide HEI2 optics are designed to improve spread, color mixing, and evenness. The broad beam can also reflect from aquarium side glass, helping reduce shadows and reach coral undersides. It cannot make physical obstructions irrelevant.

Open islands favor deliberate fixture placement

For two or more separated islands, multiple XR15 fixtures can be positioned over the occupied zones rather than centering one larger fixture over the geometric middle of the aquarium. This modular arrangement can be useful when the space between islands is intentionally open or when the islands are not evenly spaced.

An XR30 may be simpler when a broad, continuous rock structure fills the central display. Its recommended 36-by-30-inch footprint covers more area from one mounting point and reduces the number of individual fixtures, power supplies, and positions that must be arranged.

Tall or dense structures reward overlap

When colonies or rockwork create substantial self-shading, fixture count becomes a question of light direction as well as total footprint. Two smaller fixtures placed apart can send light toward a structure from different positions. One XR30 supplies broader output from a single fixture location, but it remains one overhead location.

This does not make two XR15s universally better than one XR30. A pair takes more mounting space and requires two fixture positions. The right choice depends on whether your aquascape benefits more from separated points of light or from the broader footprint of one larger unit.

Plan for the reef you intend to build

A new aquascape may have little shading, while mature branching or plating colonies can occupy much more area. If you expect coral to fill previously open gaps, plan mounting and track space so fixtures can be repositioned or added. EcoTech’s multi-light track system allows Radions to slide along a rail, making it easier to revise spacing as the layout changes.

Braces, openings, and mounting constraints

Center braces, cross braces, and canopy openings can interrupt an otherwise logical lighting position. Before choosing a model, look down at the tank from above and mark every unobstructed opening. A fixture centered over a brace is not the same as one centered over open water, so the usable mounting locations may determine the model before gallon capacity does.

The XR15’s 7-by-7-inch body and smaller recommended footprint can make it easier to assign individual fixtures to separate openings. The XR30 is 11.8 inches long and may suit a larger unobstructed opening, but its broader coverage is less useful if the ideal centerline is blocked. These are layout considerations rather than universal rules; brace dimensions and positions differ among aquariums.

Also account for the mounting system. EcoTech offers single-fixture RMS mounting options, hanging arrangements, and a multi-light track. The RMS track works with XR15 and XR30 fixtures, including a mixture of both, and track lengths are available up to eight feet. Lights can be mounted at 90-degree increments, providing another way to align a fixture with the tank’s geometry.

Both G6 sizes have a recommended mounting height of 8–10 inches. Confirm that a canopy or surrounding structure provides enough room for the fixture and intended mounting approach. Each model includes eyelet mounts, but the broader mounting arrangements are separate options described in the product records.

Uniformity versus minimum fixture count

The fewest fixtures that technically reach a display’s edges may not create the visual uniformity every reefer wants. A mixed reef with isolated coral groups may tolerate more variation between zones. A continuous coral canopy or display intended to look consistent from end to end may benefit from overlapping footprints.

Coverage asks whether light reaches an area. Uniformity asks how consistently adjacent areas are illuminated. Those are related, but they are not identical buying goals.

The XR30 is attractive when minimizing fixture count is important because its recommended footprint is 36 by 30 inches. The XR15 becomes compelling when you would rather distribute several smaller fixtures and adjust their spacing around the aquascape. Neither approach is automatically more uniform; spacing, mounting, obstructions, and coral placement control the result.

One XR30 versus two XR15s

This is a common comparison because two XR15 fixtures provide two independently positionable light sources, while one XR30 offers a larger integrated fixture. Based on the supplied specifications, two XR15s have 100 LEDs combined and a combined maximum draw of 210W. One XR30 has 100 LEDs and a maximum draw of 215W. Those totals are close, but they do not make the layouts interchangeable.

• Choose one XR30 when a continuous 36-by-30-inch recommended footprint matches the coral area and a single central fixture position is unobstructed.
• Choose two XR15s when separated islands, braces, or a long layout benefit from two adjustable positions.
• Favor the XR30 if reducing the number of fixtures and power supplies matters more than modularity.
• Favor multiple XR15s if future spacing changes or directional overlap are central to the plan.

At the listed source prices, one XR30 is $999.99 and one XR15 is $529.99, making two XR15s $1,059.98 before mounting equipment. Price should therefore be considered together with the required mounts, rail configuration, number of power supplies, and desired placement—not just the fixture count.

Practical layout examples

A compact 24-inch square coral area

A single XR15 aligns directly with a 24-by-24-inch recommended footprint when the coral-bearing area fits beneath it and there is an unobstructed central mounting position. An XR30 would provide a larger footprint, but that extra area may not be necessary. Choose the larger model only if the wider spread, future display plans, or aquascape justify it.

A 36-inch-long, wide reef

An XR30’s 36-by-30-inch recommended spread is the straightforward dimensional match for a continuous reef occupying that area. However, two XR15s can make more sense when the tank has two separated islands, a central brace, or a preference for overlapping light from distinct positions.

A 48-inch-long display

The XR30 lists a maximum spread of 48 by 30 inches, but its recommended spread is 36 by 30 inches. If coral occupies the full 48-inch length and end-to-end consistency is important, do not treat maximum spread as equivalent to recommended uniform coverage. Multiple fixtures may be the more appropriate layout. If coral is concentrated centrally with open ends, one XR30 may align more closely with the occupied area.

A long reef with repeating rock islands

Several XR15s on a track can be spaced over individual islands. XR30 fixtures can reduce the total fixture count when each broader footprint corresponds to a larger section of continuous rockwork. A mixed XR15 and XR30 rail is also supported, so an asymmetrical aquascape does not require identical fixtures at every position.

XR15 strengths and trade-offs

The XR15 is the more compact option at 7 by 7 by 1.5 inches and 3.5 pounds. Its recommended 24-by-24-inch footprint lends itself to cubes, compact coral zones, island layouts, and arrays where each fixture can be positioned independently.

Its primary trade-off is that covering a long or wide continuous reef may require more fixtures and mounting positions. Each XR15 has a maximum draw of 105W and includes its own power supply, adapter cable, eyelet mounts, microfiber cloth, and quick-start guide. More fixtures therefore mean more individual components to route and mount.

XR30 strengths and trade-offs

The XR30 measures 11.8 by 7 by 1.5 inches and weighs 4.5 pounds. Its recommended 36-by-30-inch footprint is better suited to broad, continuous coral areas where one larger fixture can replace multiple small lighting zones. Its maximum published spread is 48 by 30 inches, and maximum draw is 215W.

The trade-off is reduced positional modularity compared with two separately mounted XR15s. A brace or strongly asymmetrical aquascape can make one broad central source less convenient. The XR30 also requires a wider physical mounting area, so canopy openings and obstructions should be checked before purchase.

Blue versus Pro: choose color preference after size

Once the fixture size and count are settled, choose between Blue and Pro. The G6 Blue models prioritize a blue and actinic presentation, with tuning from actinic toward a crisper appearance. Their diode mix includes 395nm, 405nm, and 415nm UV wavelengths plus violet, blue, and royal-blue channels. EcoTech positions Blue for stronger fluorescence emphasis and blue-spectrum intensity.

The Pro models use a more balanced LED mix, including greater representation from white, red, and green channels. EcoTech describes the Pro spectrum as similar in character to metal-halide output and notes its use in public aquariums, aquaculture facilities, and coral spawning laboratories.

Blue versus Pro does not change the published recommended footprint within a given size. Both XR15 versions are rated for 24 by 24 inches of recommended spread, and both XR30 versions are rated for 36 by 30 inches. Pick XR15 versus XR30 for physical coverage first; then select Blue or Pro according to the visual and spectral balance you prefer.

A step-by-step selection method

1. Measure the display. Record length, front-to-back width, and available mounting height rather than relying on gallons.
2. Map the coral area. Mark where coral is placed or expected to grow, including elevated shelves and separated islands.
3. Mark braces and obstructions. Identify the actual open-water positions available to each fixture.
4. Compare recommended footprints. Use 24 by 24 inches for the XR15 and 36 by 30 inches for the XR30 as the primary planning figures.
5. Decide how much overlap you want. Greater emphasis on uniformity may require more overlap than a minimum-coverage plan.
6. Choose fixture geometry. Select XR15s for modular zones or XR30s for broader continuous sections. A mixture is possible on the RMS multi-light track.
7. Select Blue or Pro. Choose Blue for a blue-weighted, fluorescence-focused presentation or Pro for a more balanced spectrum.
8. Plan control and mounting. Both sizes use Mobius, support MXM integration, and have a recommended mounting height of 8–10 inches.

Final recommendation

The XR15 is not simply the choice for fewer gallons, and the XR30 is not automatically the choice for more gallons. The XR15 is the more modular lighting building block; the XR30 covers a broader continuous footprint from each fixture. A small but unusually wide tank may favor an XR30, while a larger braced display with separated islands may be better served by several XR15s.

Use recommended coverage rather than maximum reach as your starting point, then adjust for aquascape, coral location, braces, and the uniformity you want. When those factors are mapped before purchase, the XR15-versus-XR30 decision becomes a practical layout choice instead of a gallon-based guess.