THE SHORT ANSWER
For most 90cm reefs, start with two lights rather than stretching one
A 90cm tank usually needs two compact or mid-size lights, one light designed to be long and narrow, or a larger light backed up with fill lighting. One 60cm-class puck can leave you with a bright centre and dim ends, however powerful it is. How many lights you need depends on the length your corals cover and where shadows fall, not on how many litres the tank holds.
For a mixed reef, two XR15s, A360XEs or similar give a sensible starting layout with overlap in the middle. Two cheaper compact lights can work for modest coral islands. Full-width SPS may need lights closer together or extra bars, and should be checked with a PAR map.
DECISION TABLE
Which one is right for your tank?
Each island can have its own light
Use two compact or mid-size lights, and measure the overlap.
The outer 15cm at each end is open water
One wider light may work, but don’t stretch a 60cm claim without checking.
The ends and the sides of colonies need good light
Plan two 60cm-class lights and tune the overlap.
Even light and shading from growth matter most
Use several lights or add bars, and check with a full map.
01 / IN DETAIL
Why 90cm changes how you buy
At 90cm, the tank is longer than the recommended spread of many popular compact lights. Turning up the intensity doesn’t stretch the light evenly; it usually makes the centre brighter while the ends stay weak. Raising the light can widen its spread, but you lose intensity and more light spills into the room. The problem is distribution, not a lack of watts.
Start by splitting the length your corals cover into overlapping zones. If corals take up 75cm, two lights roughly centred over each half can give an overlap you can control. The exact spacing depends on the lenses, height, depth and aquascape, so treat your layout as a starting point to test.
02 / IN DETAIL
Two compact lights or two premium lights?
Two AI Prime-class lights can make good sense for two moderate islands or a soft coral and LPS reef. You get independent placement and a familiar app for less per light. Mounted, the pair can cost close to one premium light, but one premium light may still not cover a full 90cm.
Two XR15 or A360XE-class lights buy you a wider spread and more intensity headroom. That can be worth it for deeper tanks, tanks that are wide from front to back, or demanding corals across most of the tank. It can also be money wasted on an open aquascape. Match the size of light to your coral area, not to the label on the glass.
03 / IN DETAIL
Overlap is something you design
The middle of a two-light layout gets light from both. Put the lights too close and the middle becomes a hotspot while the ends go short. Too far apart and you get a dark strip down the middle. So your PAR map should include both ends, the overlap in the middle, and the front and back edges at several coral heights.
Programme the two lights as one system. Identical settings are a good start if the layout is symmetrical, but rock and corals may need small local changes. Don’t fix one shadow by turning up every channel on both lights. Moving a light, changing its height or adding a narrow fill light is often more precise.
04 / IN DETAIL
Front-to-back depth can catch you out
A shallow 90 × 35cm frag tank and a 90 × 60cm display are very different lighting jobs. In the wider tank, each light has to cover more from front to back while also overlapping along the length. Counting lights based on the 90cm length alone can underestimate what you need.
Map how wide your rockwork actually is and where you view the tank from. A central ridge can keep the coral area narrow even in a wide tank, while rock close to the front and back glass needs a wider spread. Factor in lid braces and any central overflow that limits where lights can go.
05 / IN DETAIL
Bars and fill lights solve a different problem
Extra LED bars can fill shadows at the front, back or sides without replacing your main lights. They’re especially useful once branching colonies grow and shade themselves. Give a bar a clear job, such as lighting from a different angle, adding spectrum or covering an edge, rather than adding one because the tank still looks a bit dim.
Price in the rails, brackets and any separate timers or controllers. Bars add cables and can add glare. Check what they actually add with a meter and by watching your corals, and you may need to turn the main lights down after adding fill so the total light doesn’t jump too quickly.
06 / IN DETAIL
The real three-year cost
Two lights doubles several costs comparison tables tend to leave out. Include two mounts or a rail, two power supplies, any shades, setup time and the chance of a failure. A rail makes lining lights up easier, but can make taking them off or opening the lid harder. Think about how the tank copes if one light fails.
Work out electricity from the schedule you’ll actually run across both lights. For example, two 59W lights averaging 65% over ten hours use much less than 118W at full power all day. Measure once the reef has settled, and review the schedule when coral growth changes how far the light reaches.
07 / IN DETAIL
Our shortlist
For a 90cm soft coral and LPS reef with two islands, start with two compact lights you can control. For a mixed reef across the full width, compare two XR15s, A360XEs or similar mid-size lights, and price a rail if one is available. For mostly SPS, treat two lights as the starting point and look at adding fill from an angle or extra coverage.
Don’t buy everything you might one day need on day one unless your corals need it now. A modular setup can start with two lights and add fill later, once you’ve measured. Leave room on the mounts and spare capacity on your controller, so upgrading later is planned rather than improvised.
WORKED UK EXAMPLE
A 90 × 50 × 50cm mixed reef
The aquascape covers 78cm and rises into two main shelves. The owner first considers one powerful central puck to keep the top of the tank tidy. A sketch from above shows the end shelves sit outside a safe 60cm spread, and the central rock blocks light to the lower front faces.
Instead, two mid-size lights go over the shelves, with a controlled overlap in the middle. LPS go on lower and outer spots while they settle in, and SPS wait until the tank has been mapped. The owner leaves space at the front for a fill bar but won’t buy it until growth or measurements show it’s needed. The system can grow without pretending the starting setup is final.
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
Can one XR30 light a 90cm tank?
A larger light may be worth considering, but check the exact model’s spread against your depth and coral area. One central light can still leave the ends or parts of the rock in shadow.
How many AI Primes do I need for 90cm?
Two can suit separate, moderate coral areas. Demanding corals across the full width may need bigger lights or extra fill.
How far apart should two lights be?
There’s no single answer. Start from where your corals will go and the maker’s guidance, then map the ends and the overlap in the middle.
Do both lights need to be the same model?
Matching lights make programming and blending easier, but a fill bar with a clear job can light a different angle without duplicating your main light.
Should both lights run at the same intensity?
In a symmetrical layout that’s a good start, but adjust the final settings to your measurements and where your livestock sits.
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.