THE SHORT ANSWER
Treat a 120cm reef as a lighting array, not a single product
A 120cm reef normally needs two larger fixtures, three compact-to-mid-size sources, or a purpose-built elongated/hybrid system. Two 60cm-class units are the obvious mixed-reef starting layout, but their nominal spreads only meet at a line. Useful overlap, tank width and mature coral shadowing can require tighter spacing or fill lighting.
For SPS across the full display, design around uniformity and serviceability rather than maximum central PAR. For islands with open sand, place sources over the coral zones and avoid paying to illuminate empty ends. A 120cm recommendation must include fixture count, mounts, controller capacity and combined electricity.
DECISION TABLE
Choose by the constraint that actually changes the answer
Independent centres and open water between
Three compact sources can target each island.
Most of the 120cm carries coral
Begin with two broader fixtures and test the centre overlap and ends.
Front-to-back demand compounds the length
Use a broader array or front/rear fill, not length calculations alone.
Colony growth progressively blocks side light
Design multiple angles and reserve capacity for fill.
01 / LIGHTING INTELLIGENCE
Two nominal 60cm spreads do not create a measured 120cm field
Marketing footprints describe approximate useful areas under stated or implied conditions. Placing two 60cm claims edge to edge leaves no safety margin and says nothing about the centre seam. Real arrays need intentional overlap so the centre is not dark, while the lights also remain far enough apart to serve the ends.
The correct positions depend on the coral-bearing length. If rock occupies 100cm, centre the array over that zone rather than the glass. Measure at the end shelves, central overlap, front and rear faces and several depths. Adjust spacing before solving every weakness with more intensity.
02 / LIGHTING INTELLIGENCE
Two large fixtures or three smaller fixtures
Two larger fixtures reduce cable and mounting count and can form a clean symmetrical installation. They may provide strong output and wider optics, but each remains a relatively concentrated source. Three smaller units create more angles and let the outer lights sit closer to end islands, often improving shadow control.
Three fixtures also add hardware and programming complexity. A shared rail can make alignment easier, but verify the rail span, suspension points and load. The winning layout is the one that distributes usable light with the fewest unnecessary components—not automatically the one with more watts or more pucks.
03 / LIGHTING INTELLIGENCE
A 60cm front-to-back tank needs two-dimensional planning
Many four-foot displays are also 50–65cm from front to back. An array spaced perfectly along the length can still leave the front and rear coral faces weak. Wide rock shelves and central overflows change the available mounting lines. A single row of pucks may need higher mounting, broader optics or front/rear supplementation.
Use a top-down drawing with both axes. Mark the coral zone, glass braces, overflow and lid openings. Then draw conservative fixture rectangles and their overlap. This simple map exposes whether the proposed count only solves length while ignoring width.
04 / LIGHTING INTELLIGENCE
Design for coral growth, not the empty launch tank
New frags create little self-shadowing, so an initially sparse reef can appear evenly lit. As branching and plating corals grow, their upper surfaces intercept more light and their lower or inward faces receive less. A mature SPS plan needs multiple angles or a realistic way to add fill without rebuilding the canopy.
Preserve rail space, sockets and controller capacity. Keep access for cleaning lenses and fans. A beautiful fixed mount that cannot be moved when colonies grow is a fragile design. Modularity is an ownership feature, not merely an enthusiast upgrade.
05 / LIGHTING INTELLIGENCE
Electrical and thermal planning
A multi-light array can become one of the aquarium’s largest controllable loads. Add maximum rated wattage to understand circuit and heat exposure, then calculate energy from the actual programme. Do not forget bars, fans and control accessories. Use properly protected sockets, drip loops and manufacturer-approved power supplies; fixed household electrical changes belong with a competent electrician.
The room absorbs the consumed energy as heat. In summer, several high-power fixtures can add to cooling pressure, while in winter some heat may reduce heater duty. Do not subtract an assumed heater saving from the lighting bill without measurement. Monitor room and water temperature through seasonal changes.
06 / LIGHTING INTELLIGENCE
Ownership and failure tolerance
Multiple fixtures provide partial light if one unit fails, but only if the remaining schedule and positions can support a temporary reduced plan. Save configuration files and photograph the installation. Check whether a failed power supply can be sourced in the UK without replacing the light.
A rail or hanging system should allow one unit to be removed without dismantling the whole canopy. Route cables so salt creep and water cannot reach connectors. These service details matter more over five years than another app effect used only during setup.
07 / LIGHTING INTELLIGENCE
The Reef Bench shortlist
For three distinct islands, compare three Prime-, ReefLED- or similar compact sources with a common mounting plan. For continuous mixed reef, compare two larger broad-coverage fixtures against three smaller sources. For full SPS and wide tanks, include bars or a second lighting row in the design and verify the mature-growth margin.
The exact product follows the layout. Choose the ecosystem with supported UK mounts, recoverable control and replacement parts, then confirm the array through measurement. A four-foot tank rewards system thinking; isolated product rankings are too small for the decision.
WORKED UK EXAMPLE
A 120 × 60 × 55cm SPS-leaning reef
The aquascape runs for 105cm and is 48cm wide, with branching coral planned on both front and rear ridges. Two central fixtures can cover the length on paper, but the ridges shade one another and the outer shelves sit close to the boundaries. Raising the fixtures improves spread while creating glare in the adjacent dining area.
The owner compares two large fixtures plus front fill bars with three mid-size sources on a rail. The three-source route provides more placement angles and avoids relying on maximum output. The rail is positioned so each light can be removed, and sockets are reserved for future fill. A baseline PAR map is recorded before livestock, then repeated after major colony growth rather than copied from another four-foot aquarium.
THE REEF BENCH CHECK
What this page can—and cannot—prove
Electrical input, physical dimensions, published spread, control method and included accessories are facts that can be checked. They help eliminate unsuitable choices and expose the complete installed cost. They do not prove the PAR at a coral, long-term colour response, fan noise in your room or reliability over several years. Those outcomes depend on the individual unit, installation, programme, water, aquascape and maintenance.
Before changing a working aquarium, record the present fixture, mounting height, schedule, channel intensities and livestock positions. Photograph the tank under the same settings and, where coral demand justifies it, borrow or rent a suitable PAR meter. Change one meaningful variable at a time, begin conservatively and use acclimation controls. A copied online schedule is a starting hypothesis, not evidence that the same percentages will suit another aquarium.
Our recommendations therefore use conditional language. “Best for” means the product or layout solves the stated constraint more convincingly on documented evidence; it does not mean every buyer should choose it. A lower-cost fixture can be the stronger decision for a compact soft-coral island, while a higher-cost system becomes defensible when broader distribution, multiple-light coordination or future expansion is a real requirement.
OWNERSHIP COST
Compare the complete mounted system
Add the fixture, arm or hanging kit, rail, diffuser or shade, controller where required, and any second light needed for the intended footprint. Then calculate electricity from the programmed output and photoperiod rather than assuming the label maximum runs all day. A premium light may justify its cost through distribution or control; it does not justify itself merely by having a higher wattage or a longer feature list.
Also price the parts most likely to interrupt ownership: power supplies, fans, mounting hardware and the route for UK warranty support. App-based control is convenient, but confirm how the fixture behaves after a power cut, router change or service outage. Save the manual and final schedule outside the app. The best installation remains understandable and recoverable when the phone or internet is unavailable.
COMMON QUESTIONS
Frequently asked questions
How many reef lights for a 120cm tank?
Usually two larger or three smaller sources, but coral-bearing length, tank width, depth and aquascape decide the final array.
Are two XR15 lights enough?
They are a plausible mixed-reef starting point for some 120cm layouts, but nominal 60cm spreads need measured overlap and end checks.
Should I use an XR30 instead?
Compare the exact larger-fixture spread and count as a complete array. One high-output unit is not a general solution for 120cm length.
Do LED bars replace main lights?
Usually they are best used as defined fill or supplementation unless the bar system is explicitly designed as the primary source.
When should I repeat PAR measurements?
After changing height, schedule, optics, aquascape or when substantial coral growth changes shadowing.
PRIMARY SOURCES
Check the specifications yourself
Links below lead to manufacturer information used for the factual comparison. Coverage remains a manufacturer claim unless explicitly described as our measurement.