THE SHORT ANSWER
PAR measures how much usable light arrives, not how good that light is
PAR stands for photosynthetically active radiation: light between roughly 400 and 700nm. A PAR meter actually reports PPFD, the amount of that light reaching one spot each second, usually in micromoles per square metre per second. For reefers, it’s useful for comparing spots in the tank, finding hotspots and shadows, and checking things again after you change height or output.
PAR doesn’t tell you the exact spectrum, whether every wavelength is equally useful to corals, how well a coral has adjusted, or whether it will grow and colour up. Use it alongside spectrum information, advice for the specific coral, stable water and what you see your corals doing. A map of the tank is worth far more than one impressive reading in the middle.
DECISION TABLE
Which one is right for your tank?
Makers measure their maps in different conditions
Compare the test setups before comparing the numbers.
You know the reading at that spot and what the coral is used to
Start cautiously, and move it or change intensity gradually.
Nearby spots are fine but one face is in shadow
Change the angle or add fill rather than turning up every channel.
Height, water clarity or schedule has changed
Note the new conditions and compare like with like.
01 / IN DETAIL
PAR, PPFD and the number on the meter
When reefers say ‘PAR’, they usually mean the PPFD reading on a meter. The meter counts the light in its range that reaches the sensor each second. That reading belongs to one exact spot, angle and moment. Move the sensor a few centimetres under a puck, into a shadow or under a ripple, and the number changes.
Write the unit next to the reading, and note whether the pumps were on, what the light was set to, how high it was mounted and how deep the sensor was. Without that, nobody can repeat the reading. A screenshot of a colour map is only useful if the scale and the conditions go with it.
02 / IN DETAIL
Why one reading in the middle misleads
The spot directly under a compact LED usually gets the most light. Publishing only that number hides how quickly it drops towards the edges and behind rock. Corals live on surfaces all over the tank, not at the exact centre of the glass. A useful map measures the spots where your livestock will actually sit.
Measure in a grid across the area your corals cover, and repeat it at the heights that matter. In a complicated aquascape, include the front, sides and shaded faces. You’re not aiming for perfectly even light, because different animals need different zones. You’re aiming to know where those zones are, and whether sharp hotspots are creating risks you could avoid.
03 / IN DETAIL
PAR doesn’t describe spectrum
Two lights can give the same PPFD reading while spreading their light very differently across violet, blue, green and red. Corals and the algae living in them don’t respond the same way to every colour, and the colours you like to look at add another layer. So a PAR number alone can’t tell you which spectrum is better.
Use the maker’s spectrum information and good care advice, then keep your chosen schedule steady long enough to judge it. Don’t turn channels up just because a blue-heavy tank looks dim to you. Our eyes are less sensitive to those colours, so a tank can look dimmer than it really is.
04 / IN DETAIL
Coral PAR ranges are a guide to placement, not a prescription
Charts online often give broad PAR ranges for soft corals, LPS and SPS. They can help you plan a first map, but each group includes animals from different places, with different shapes and different histories. A coral that has lived under low light can be stressed if you move it straight into the ‘right’ range from a chart.
Find out what light it’s used to if you can, start at the cautious end, and watch it for a sensible length of time. Pale tissue, closing up and other signs of stress can have many causes, so don’t blame the light from one symptom while you haven’t checked salinity, temperature, nutrients and pests.
05 / IN DETAIL
How to take a useful PAR map
Set your normal schedule to the point you want to measure and let the light reach it. Set the pumps the way you’ll set them every time. Hold the sensor level where your corals will sit, without shading it with your hand. Write the readings on a labelled grid rather than relying on memory.
Measure again after big changes: a new light, a different mounting height, a new lens or diffuser, a rebuilt aquascape or a lot of coral growth. If you’re borrowing a meter, plan your grid before the day and save every setting. The point is to make future changes comparable, not to chase one target number.
06 / IN DETAIL
What to do with the results
A single low reading might be solved by moving a coral or adding light from another angle. A small, bright hotspot might be fixed by mounting higher, adding a diffuser, turning the output down or moving demanding corals away from the exact centre. Low readings everywhere can justify more output, but only if the light has headroom to spare and the spectrum and schedule still suit your tank.
Change one thing at a time and measure again. If you turn up the intensity and lower the light at the same time, you can’t tell which change did what. Use acclimation modes or small manual steps, and stop if your livestock shows clear distress.
07 / IN DETAIL
How we read PAR claims
We treat a maker’s PAR map as evidence from their own test setup, not as our own test. Where we can, we note the tank size, depth, mounting height and schedule. A map with its conditions shown is more useful than a bigger number without them.
When the evidence is incomplete, we recommend more cautiously: a smaller coverage area than claimed, more margin at the edges and more emphasis on measuring. That keeps the uncertainty in plain view rather than hiding it behind numbers that look precise.
WORKED UK EXAMPLE
Mapping a 60cm mixed reef
A meter reads 260 right under the light on the upper shelf, 150 in the middle at mid height, 95 on the lower outer shelves and 55 behind the right-hand island. The centre reading alone would make the setup look strong, but the grid shows one specific shadow caused by the rock.
Instead of turning the whole light up, the owner moves the affected low-light coral slightly forward and angles a small fill light at the back face. The main schedule stays the same while they watch. They repeat the map with the same pump settings and schedule, and can see the change helped the weak spot without making the upper shelf an even bigger hotspot.
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
What PAR do corals need?
There’s no single number. Use broad guidance for the species, what the coral is used to and measured placement, then watch how it does in stable water.
Can I measure reef PAR with my phone?
Phone sensors and apps aren’t a substitute for a proper calibrated underwater sensor. Treat their readings with caution.
Should the pumps be on when I measure?
Pick one setting and always use it. Surface movement can make readings jump around, but your tank normally runs with that movement.
Does higher PAR mean faster growth?
Not necessarily. Too much light can stress corals, and growth also depends on spectrum, water chemistry, nutrients, flow and the coral itself.
How often should I map PAR?
After real changes to your lighting or aquascape, and when growth changes where shadows fall. It’s not a number to chase every week.
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.