THE SHORT ANSWER
For most 90cm reefs, begin with two light sources rather than stretching one
A 90cm tank normally needs two compact or mid-size fixtures, one deliberately elongated system, or a larger fixture supported by fill lighting. One 60cm-class puck may create a bright centre and dim ends even when its maximum output is high. The number of sources is driven by the coral-bearing length and shadow geometry, not by nominal tank volume.
For a mixed reef, two XR15-, A360XE- or similar coverage-class units provide a sensible planning layout with overlap. Two lower-cost compact fixtures can work for restrained coral islands. A full-width SPS plan may need closer spacing or bar supplementation and should be verified with a PAR map.
DECISION TABLE
Choose by the constraint that actually changes the answer
Each island can have its own centre of light
Use two compact or mid-size sources with measured overlap.
The outer 15cm at each end is open
One broader source may be plausible, but do not stretch a 60cm claim blindly.
Ends and colony sides need useful exposure
Plan two 60cm-class units and tune the overlap.
Uniformity and growth shadowing dominate
Use multiple sources or hybrid bars and verify a complete map.
01 / LIGHTING INTELLIGENCE
Why 90cm changes the buying method
At 90cm, the tank is longer than the recommended spread of many popular compact fixtures. Increasing intensity does not extend a field evenly; it often makes the centre brighter while the ends remain weak. Raising the fixture can widen the field, but trades away intensity and adds room spill. The problem is distribution, not simply insufficient wattage.
Begin by dividing the coral-bearing length into overlapping zones. If coral occupies 75cm, two sources centred roughly over each half can create a controllable overlap. Exact spacing depends on optics, height, depth and aquascape, so the layout remains a starting hypothesis.
02 / LIGHTING INTELLIGENCE
Two compact fixtures versus two premium fixtures
Two AI Prime-class fixtures can be a rational route for two moderate islands or a soft/LPS reef. They provide independent placement and a familiar app ecosystem at a lower per-unit price. Their combined mounted cost can approach one premium fixture, but one premium fixture may still fail the full 90cm geometry.
Two XR15 or A360XE-class units purchase more distribution and intensity headroom. That can be justified for deeper tanks, wider front-to-back dimensions or demanding coral across most of the display. It can also overspend on an open aquascape. Match the source class to the coral area, not the glass badge.
03 / LIGHTING INTELLIGENCE
Overlap is a design variable
The centre of a two-light layout receives energy from both fixtures. If the units are too close, the overlap becomes a hotspot while the ends stay under-served. If too far apart, a dark central seam appears. A PAR map should therefore include the ends, centre overlap and front and rear edges at several coral heights.
Programme the fixtures as one system. Identical settings are a useful start when the layout is symmetrical, but rock and coral may require local adjustment. Avoid correcting one shadow by increasing every channel on both lights. Repositioning, changing height or adding a narrow fill source can be more precise.
04 / LIGHTING INTELLIGENCE
Front-to-back width can be the hidden constraint
A 90 × 35cm shallow frag-style aquarium and a 90 × 60cm display are not the same lighting job. The wider tank asks each source to cover more front-to-back area while also overlapping along the length. A product count based only on 90cm can therefore understate the requirement.
Map the actual rock width and viewing priorities. A central ridge may keep the coral zone narrow even in a wide tank, whereas rock near the front and back glass needs a broader field. Include lid braces and any central overflow that affects mounting position.
05 / LIGHTING INTELLIGENCE
Bars and hybrid fill solve a different problem
Supplementary LED bars can fill front, rear or side shadows without replacing the main sources. They are particularly useful as branching colonies grow and create self-shadowing. The bar should have a defined job—fill angle, spectral support or edge coverage—rather than being added because the tank still looks visually dim.
Price rails, brackets and independent timers or controllers. Bars add cables and can increase glare. Their contribution should be checked with measurement and coral response, and the main fixtures may need to be reduced after fill is added to avoid raising total exposure too quickly.
06 / LIGHTING INTELLIGENCE
The real three-year cost
A two-light system doubles several items that comparison tables often omit. Include two mounts or a rail, two power supplies, potential shades, programme setup and replacement risk. A rail can simplify alignment but may make removal or lid access harder. Check how one failed unit affects the remaining light field.
Electricity should use combined programmed duty. For example, two 59W fixtures at an effective 65% for ten hours are not the same as 118W at full output all day. Measure once the reef is stable and review the schedule after coral growth changes the usable field.
07 / LIGHTING INTELLIGENCE
The Reef Bench shortlist
For a 90cm soft/LPS reef with two islands, start with two compact controllable fixtures. For a full-width mixed reef, compare two XR15, A360XE or equivalent mid-size sources and price a rail where available. For SPS-dominant layouts, treat two sources as the beginning of the design and evaluate angled fill or additional coverage.
Do not buy all planned expansion on day one unless the coral plan requires it. A modular system can start with two sources and add measured fill later. Preserve mounting space and controller capacity so the upgrade remains deliberate rather than improvised.
WORKED UK EXAMPLE
A 90 × 50 × 50cm mixed reef
The aquascape occupies 78cm and rises into two main shelves. The owner first considers one high-output central puck to keep the top clean. A top-down sketch shows that the end shelves sit outside a conservative 60cm field, while the central rock blocks light to the lower front faces.
Two mid-size fixtures are placed over the shelves with controlled centre overlap. LPS occupy lower and outer positions during acclimation, and SPS placement waits for a map. The owner leaves front mounting space for a future fill bar but does not buy it until growth or measurement demonstrates the need. This makes the system expandable without pretending the launch configuration is permanent.
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
Can one XR30 light a 90cm tank?
A larger fixture may enter the shortlist, but the exact model, spread, depth and coral zone must be checked; one central source can still create end or structural shadows.
How many AI Primes for 90cm?
Two can suit separated moderate coral zones; demanding full-width coverage may require a different source class or supplementation.
How far apart should two lights be?
There is no universal spacing. Start from the intended coral zones and maker guidance, then map the ends and centre overlap.
Do I need identical fixtures?
Matching units simplify programming and blend, but a defined supplementary bar can solve a different angle without duplicating the main light.
Should both lights use the same intensity?
A symmetrical layout can start there, but final settings should respond to measured distribution and livestock placement.
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.