THE SHORT ANSWER
Start with the maker’s range, then fine-tune with a PAR map
Mounting a reef light higher usually spreads the light wider and blends the colours better, but lowers the peak intensity and sends more light into the room. Mounting it lower concentrates the light and cuts spill, but can create hotspots and harder shadows, and puts the light closer to salt spray. There’s no single ideal height.
Use the mount made for your exact light and the maker’s starting guidance. Always measure from the water surface to the same point on the light, note the depth from the waterline to your corals separately, and map your coral area before changing intensity. Tune height and output together, rather than copying either number from someone else.
DECISION TABLE
Which one is right for your tank?
The light is too concentrated
Raise it within the maker’s range or add another light, then measure again.
You can see the LEDs below the shade line
Lower it safely, add an approved shade or change the viewing angle.
The colours don’t blend before they reach the water
Raise it within the maker’s range and compare the results.
The light is too exposed, or the surface is very choppy
Restore a safe gap, and check the maker’s cleaning and corrosion advice.
01 / IN DETAIL
Two distances people mix up
Mounting height usually means the distance from the water surface to a set point on the light, though makers don’t all define it the same way. Coral depth is the distance from the water surface down to your corals. Add the two together and you get a rough idea of how far the light travels, before refraction and geometry come into it.
Write both down in centimetres, and note your tank’s normal water level. In an open tank, evaporation changes the distance to the surface; an auto top-off keeps it steady. A ‘20cm high’ tip copied from a forum is useless if you don’t know whether it means above the rim, the water or the coral.
02 / IN DETAIL
Higher doesn’t simply mean better coverage
Raising a light makes the lit area bigger, but the same output is spread more thinly and more of it escapes past the glass. The edges may get more even, while every point gets less light. A high mount can also put glare in people’s eyes and light up the ceiling or walls.
Use height to shape the light, not to rescue a light that’s simply too narrow for the tank. If you can only get acceptable spread by mounting it impractically high, with glare, a second light or a different lens is the better fix.
03 / IN DETAIL
Lower doesn’t simply mean more PAR
Lowering a light can raise the intensity in the middle, but it can make a hotspot worse and cause light to drop off sharply around rock. It also brings the light closer to salt spray and makes it harder to work in the tank. Some mounts and lenses have minimum gaps you shouldn’t ignore.
A low light can work over a compact island where you don’t want spill, as long as the colours still blend and you’re in control of the coral area. Turn the output down while you make changes, and measure again rather than assuming the same schedule is safe at the new height.
04 / IN DETAIL
Glare and the living room test
Many reef tanks sit next to a sofa, a dining table or a desk. A light that looks tidy when you’re standing can show bright LEDs when you sit down. Before drilling holes or buying a rail, hold a piece of cardboard at the planned height and look at it from where people normally sit and from the doorways.
Approved shades and diffusers can make the tank more comfortable to look at and blend the colours better, but they may change intensity and how the light handles heat. Include them in your cost, and measure again after fitting them. Don’t improvise with materials close to a hot light, or block its ventilation.
05 / IN DETAIL
Mounts, lids and getting at things
A rear arm needs a deep enough clamp, the right glass thickness and space clear of the filter chambers. Hanging kits need a secure fixing point that can take the weight. Rails should let you move or take off a light without taking the whole setup apart. Follow the maker’s instructions for its own hardware, rather than swapping in fixings you haven’t checked.
Leave room to open lids, empty the skimmer cup and clean the lens. Give every cable a drip loop and keep power connections away from water. If your setup needs changes to your home’s fixed wiring, use a qualified electrician.
06 / IN DETAIL
A step-by-step way to tune it
Install the light at the maker’s recommended starting height. Run a cautious schedule, map your coral area, and note any glare and spill. If the middle is too bright and the edges weak, change the height a little and repeat the same grid before turning the output up. If a shadow from the rock won’t go away, try moving the light or adding another, rather than going higher and higher.
Once you’ve settled on a height, tighten the mount, save your schedule and take a photo with a ruler in shot. Use acclimation settings when changing intensity, lenses or height with livestock in the tank. Look at it again once coral growth changes how the light falls.
07 / IN DETAIL
Our rule
Mounting height is part of how a light performs. A review that compares lights without saying how they were mounted and at what height leaves out a big part of the picture. So we compare the light as a complete package with its intended mount, and treat third-party accessories as a different setup.
The best height isn’t the one that gives the biggest reading in the middle. It’s a safe position you can maintain, giving usable light across your coral area, with acceptable spill into the room and headroom left to adjust.
WORKED UK EXAMPLE
Fixing a hotspot over a 50cm cube
A compact light sits 12cm above the water. The top of the rock in the middle reads much higher than intended, the lower outer shelves are fairly dim, and you can see colour banding. The owner first thinks about turning all the channels down, but that would make the outer shelves even dimmer.
Instead, they raise the light within the approved mounting range and repeat the same grid. The middle drops, the edges improve and the colours blend better, but there’s more glare at sofa level. They add an approved shade and measure again, since the shade can change how much light gets through. Then they adjust the output carefully. The final height is written down as a setting for this tank, not remembered as a rule for that light.
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 high should an AI Prime be mounted?
Use the maker’s current starting guidance (or that of an approved mount) for your exact model, then fine-tune for your tank size and measurements.
Does raising a light reduce PAR?
Generally, yes: peak intensity at any one point drops while the light spreads wider. Exactly how much depends on the lens and your setup.
Can I mount a reef light close to a lid?
Check the maker’s guidance on heat, condensation, materials and minimum gaps. Lids can also change reflections, salt build-up and how easy maintenance is.
Will a diffuser fix shadows?
It can improve blending and soften the light, but it can’t always make up for a missing angle of light in your aquascape.
Can I change the height with corals in the tank?
Yes, with small, careful changes. Turn the intensity down where it makes sense, use acclimation settings and watch your corals.
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.