THE SHORT ANSWER
Plan a 120cm reef as a set of lights, not a single product
A 120cm reef usually needs two larger lights, three compact or mid-size lights, or a system built for long tanks. Two 60cm-class lights are the obvious starting point for a mixed reef, but on paper their spreads only just meet in the middle. Real overlap, the tank’s width and the shade cast by grown-in corals can all mean closer spacing or fill lighting.
For SPS across the whole tank, plan for even light and easy maintenance rather than the highest PAR in the middle. For islands with open sand between them, put lights over the corals and don’t pay to light empty ends. Any recommendation for 120cm has to include the number of lights, mounts, controller capacity and the combined electricity.
DECISION TABLE
Which one is right for your tank?
Each has its own centre, with open water between
Three compact lights can target one island each.
Corals cover most of the 120cm
Start with two wider lights, and test the overlap in the middle and the ends.
Front-to-back depth adds to the length problem
Use a wider set of lights or front and back fill, not length alone.
Growing colonies gradually block light from the side
Light from several angles, and leave room to add fill.
01 / IN DETAIL
Two 60cm spreads don’t make a measured 120cm
Advertised coverage is a rough guide to the useful area under the maker’s test conditions. Putting two 60cm claims edge to edge leaves no margin and tells you nothing about the middle. Real setups need deliberate overlap so the centre isn’t dark, while keeping the lights far enough apart to reach the ends.
Where the lights go depends on the length your corals cover. If rock takes up 100cm, centre the lights over that, not over the glass. Measure the end shelves, the overlap in the middle, the front and back faces and several depths. Adjust the spacing before you try to fix every weak spot with more intensity.
02 / IN DETAIL
Two large lights or three smaller ones?
Two larger lights mean fewer cables and mounts, and a tidy, symmetrical setup. They can be powerful with wide lenses, but each is still a fairly concentrated source. Three smaller lights give you more angles, and the outer two can sit closer to the end islands, which often helps with shadows.
Three lights also mean more hardware and more programming. A shared rail makes lining them up easier, but check the rail’s length, hanging points and how much weight it can take. The best layout spreads usable light with the fewest unnecessary parts, not the one with the most watts or the most pucks.
03 / IN DETAIL
A 60cm-deep tank needs planning in two directions
Many four-foot tanks are also 50–65cm from front to back. Lights spaced perfectly along the length can still leave the front and back coral faces short of light. Wide rock shelves and central overflows limit where lights can go. A single row of pucks may need mounting higher, wider lenses, or extra lights at the front and back.
Sketch the tank from above, in both directions. Mark the coral area, glass braces, overflow and lid openings. Then draw a cautious coverage area for each light and where they overlap. This simple map shows whether your plan only solves the length and forgets the width.
04 / IN DETAIL
Plan for grown-in corals, not the empty tank
New frags cast little shade, so a sparse new reef can look evenly lit. As branching and plating corals grow, their tops catch more light and their lower or inner faces get less. A mature SPS tank needs light from several angles, or a realistic way to add fill without rebuilding the whole setup.
Leave space on the rail, spare sockets and room on your controller. Keep access to clean lenses and fans. A beautiful fixed mount that can’t be moved as colonies grow is a fragile design. Being able to adapt is a practical feature, not just an upgrade for enthusiasts.
05 / IN DETAIL
Electrical and heat planning
A set of lights can become one of your tank’s biggest electrical loads. Add up the maximum wattage to understand the load on your circuit and the heat produced, then work out energy use from the schedule you’ll actually run. Don’t forget bars, fans and control accessories. Use properly protected sockets, drip loops and the maker’s approved power supplies. Any changes to your home’s fixed wiring are a job for a qualified electrician.
All the energy used ends up as heat in the room. In summer, several powerful lights add to cooling problems, while in winter some of that heat may mean your heater runs less. Don’t knock an assumed heater saving off your lighting bill without measuring it. Keep an eye on room and water temperature as the seasons change.
06 / IN DETAIL
Maintenance and what happens when one fails
With several lights, you still have some light if one fails, but only if the others can cope with a temporary reduced schedule from where they are. Save your settings and take photos of the setup. Check whether you can get a replacement power supply in the UK without replacing the whole light.
A rail or hanging system should let you remove one light without taking down the lot. Route cables so salt creep and water can’t reach the connectors. Over five years, these details matter more than another app effect you only use while setting up.
07 / IN DETAIL
Our shortlist
For three separate islands, compare three Primes, ReefLEDs or similar compact lights on a shared mounting plan. For a mixed reef running the full length, compare two larger wide-coverage lights against three smaller ones. For full SPS and wide tanks, include bars or a second row of lights in your plan, and check you’ll have enough margin once corals grow.
The layout decides the product, not the other way round. Choose a system with UK-supported mounts, settings you can restore and available spare parts, then confirm the setup by measuring. A four-foot tank rewards thinking about the whole system; ranking individual lights is too narrow for this decision.
WORKED UK EXAMPLE
A 120 × 60 × 55cm SPS-leaning reef
The aquascape runs for 105cm and is 48cm wide, with branching corals planned on both a front and a back ridge. Two central lights cover the length on paper, but the ridges shade each other and the outer shelves sit close to the edge of the coverage. Raising the lights improves the spread but creates glare in the dining area next door.
The owner compares two large lights plus front fill bars against three mid-size lights on a rail. Three lights give more angles to work with and don’t rely on running at maximum output. The rail is placed so each light can be taken off, and sockets are kept free for future fill. A baseline PAR map is taken before adding livestock and repeated after major coral growth, rather than copied from someone else’s four-foot tank.
WHAT WE CAN AND CAN’T TELL YOU
What this comparison can prove, and what it can’t
Power draw, dimensions, published spread, controls and what’s in the box are facts we can check. They help rule out the wrong lights and show the true cost once installed. They can’t prove the PAR at a particular coral, how colours develop over time, how loud the fan will be in your room, or how reliable it will be over several years. Those depend on the individual light, how it’s installed and programmed, your water, aquascape and maintenance.
Before changing a light on a tank that’s doing well, write down your current light, its mounting height, schedule, channel settings and where each coral sits. Take photos under the same settings and, if your corals are demanding, borrow or rent a PAR meter. Change one thing at a time, start cautiously and use the acclimation mode. A schedule copied from the internet is a starting point, not proof the same percentages will suit your tank.
That’s why our recommendations are conditional. “Best for” means the light handles that situation best on the evidence available, not that everyone should buy it. A cheaper light can be the smarter choice for a small soft-coral island, while a pricier system earns its place when you genuinely need wider coverage, several lights working together or room to expand.
RUNNING COSTS
Compare the full cost once it’s mounted
Add up the light, the arm or hanging kit, any rail, diffuser or shade, a controller if it needs one, and a second light if your tank needs it. Then work out the electricity from the output and hours you’ll actually programme, rather than assuming it runs at full power all day. A premium light can be worth it for better spread or control, but not just for higher wattage or a longer feature list.
Also price the parts most likely to fail: power supplies, fans and mounts, and check how UK warranty claims work. App control is convenient, but find out what the light does after a power cut, a new router or an app outage. Keep the manual and your final schedule somewhere outside the app, so you can recover your setup without your phone or the internet.
COMMON QUESTIONS
Frequently asked questions
How many reef lights do I need for a 120cm tank?
Usually two larger or three smaller lights, but the length your corals cover, the tank’s width and depth, and your aquascape decide the final setup.
Are two XR15s enough?
They’re a reasonable starting point for some 120cm mixed reefs, but their 60cm spreads need measured overlap and checks at the ends.
Should I use an XR30 instead?
Compare the larger light’s exact spread, and how many you’d need, as a complete setup. One powerful light isn’t a general answer for a 120cm tank.
Can LED bars replace main lights?
Usually they work best as fill or extra coverage, unless the bar system is specifically designed to be the main light.
When should I repeat PAR measurements?
After changing height, schedule, lenses or aquascape, or when coral growth changes where shadows fall.
SOURCES
Check the specifications yourself
These links go to the manufacturer information we used. Coverage figures are the maker’s claims unless we say we measured them.