How to match a light in five steps
Note the inside length, front-to-back width and water depth in centimetres, not the tank’s litres.
Mark where corals will actually sit. A single central island needs different light from rockwork running wall to wall.
Compare the maker’s recommended spread, mounting height and depth, not just the biggest “up to” number.
Branching corals and tall rockwork cast more shade. A second light can help, even if one has enough power.
Add mounts, rails, controllers and realistic electricity. Check the UK warranty and where spare parts come from.
Start low, use the acclimation mode, watch how your livestock responds, and take a PAR reading when your corals need it.
A starting point for each tank size
| Tank size | Where to start | Type of light | Watch out for |
|---|---|---|---|
| 40–45cm cube | One compact light is the usual starting point | AI Prime 16HD or ReefLED G2 60 type | Overhanging rock and corals near the edges can still end up in shade |
| 50–60cm cube | One wider light may work; check the depth and where corals will go | Reef Flare Pro S, Kessil A360XE or XR15 type | A quoted 60cm spread doesn’t mean even SPS coverage across it |
| 90cm long tank | Plan for two compact lights, or one proven long fixture | Two lights give you flexibility on placement | Check the overlap in the middle and dim spots at the ends before buying |
| 120cm long tank | Usually two bigger lights, or several on a rail | Budget for the full mount and cable route | One powerful light rarely covers end to end evenly |
Why water depth changes the answer
Deeper water puts more distance between the light and your corals. But just buying more watts can create harsher hotspots near the surface. Use the maker’s actual test conditions where you can: Red Sea, for example, publishes the mounting height, tank size and readings for its G2 lights. That’s far more useful than a bare “suits 200 litres” label.
Why we don’t promise exact results
Water clarity, surface movement, mounting height, lenses, spectrum, intensity and rockwork all change how much light reaches your corals. That’s why we treat coverage as the maker’s claim and never turn watts into PAR. Two lights can use the same power and spread it very differently.
THE SHORT ANSWER
Match the area and depth before you worry about watts
A reef light has to cover your tank’s length and front-to-back width at the height you mount it, while giving the right intensity where your corals sit. Wattage and ‘suits X litres’ ratings can’t describe that. A strong central hotspot can still leave the edges and lower rockwork in the dark.
Start with your tank’s inside footprint, water depth and aquascape. Compare the makers’ coverage maps at a stated mounting height, and plan to check with a PAR meter if your livestock is demanding or needs a narrow intensity range. On a wide tank, two lower-powered lights can give more even coverage than one powerful light in the middle.
STEP BY STEP
How to do it
- 01
Measure the water footprint
Note the inside length, front-to-back width and water depth at your normal water level. Note braces, lids and overflows that affect mounting or spread.
- 02
Map out your corals
Mark the high, middle and low rock positions and the corals you’re likely to keep. The aim is good light where your animals live, not even light on bare glass.
- 03
Compare coverage fairly
Check the mounting height, lens or diffuser and intensity behind every published map. A coverage claim without those details isn’t a measurement.
- 04
Start low and check
Begin with a cautious schedule, measure where you can and acclimatise gradually. Adjust placement before turning every channel up.
YOUR SITUATION
What to do in your situation
One light covers both directions with power to spare
One compact light may be enough, if the edges get enough light.
Coverage drops at the edges, or dense rock casts shadows
Use two lights or LED bars rather than forcing one hotspot to do it all.
You need high, even intensity across growing colonies
Prioritise measured coverage, flexible mounting and lights that handle heat well.
COMMON MISTAKES
Mistakes to avoid
Using watts per litre
Power ignores optics, spectrum, mounting height, depth and where your corals actually are.
Copying someone else’s schedule
The same percentage gives different results with a different light, height, aquascape or water clarity.
Forgetting the mount
An arm, hanging kit or diffuser changes the price, position, spread, glare and sometimes the usable power.
WORKED EXAMPLE
Lighting a 60 × 45 × 45cm mixed reef
The tank is only 60cm long but quite deep from front to back. One compact light may claim enough coverage on paper, but a central rock island shades the lower back, and the front corners fall outside its strongest area. Turning up the power just makes the hotspot brighter without fixing the shadows.
Two smaller lights, a wider panel or a main light plus an LED bar can give more usable coverage. The buyer should compare the cost with mounts included and check where their corals will sit, then start below the final setting and acclimatise gradually.
BEFORE YOU CHANGE ANYTHING
Write down where you’re starting from
Note the date, the equipment or product you’re using, how you measured and the conditions behind every important number. Say whether a volume is the tank’s label figure or your real net water; whether a flow is the box rating or what actually comes out; and, for test results, the salinity, test kit and time of day. A precise-looking number is useless once you’ve forgotten where it came from.
Change one thing at a time, and decide in advance what improvement will look like: a steady test trend, less detritus, a more stable temperature, a quieter tank or a lower running cost. One reading, or one good day, isn’t a trend, and a setting copied from someone else’s tank isn’t proof it suits yours. The free Reef Companion keeps your tests, maintenance and changes in one dated log, so you can see cause and effect.
MAKING CHANGES SAFELY
Protect your tank while you make the change
Take a baseline
Measure before you install equipment, change your chemistry or alter your routine. Take photos if placement matters, and note how your livestock behaves. Without a starting point, you can’t be sure what caused any difference later.
Start small
Begin with a cautious setting or dose, and keep a way back. Hang on to the old equipment until the new one has proved itself, keep your previous schedule, and don’t add livestock at the same time as a big lighting, flow or chemistry change.
Give it the right amount of time
Mechanical problems show up straight away; nutrient and coral responses can take days or weeks. Judge the change over a sensible period, but stop immediately if you see animals in distress, a leak, overheating or an electrical risk.
Afterwards, note the result and decide whether to keep, adjust or undo the change. If the result is unclear, go back to your last stable setup and measure better, rather than piling on another change.
SIX MONTHS ON
Check it’s still the right decision
What made sense on day one can stop working as livestock grows, the room warms up, equipment gets dirty or a routine turns out to be unrealistic. Set a reminder to review it in six months, comparing what you assumed with the reality: water volume, electricity, consumables, noise, cleaning time and test trends.
Test trends, real flow, temperature, light coverage or whatever the change was for
Is it still doing its job, with room to spare?
How often you clean it, how hard it is to reach, calibration drift and missed tasks
Can you keep this up without cutting corners?
Electricity, reagents, filters, spare parts, warranty and downtime
Would you make the same choice knowing the real cost?
If it’s working, you don’t need to replace it; the review simply gives you a reason to keep it, tune it or plan an upgrade before something fails.
KNOW THE LIMITS
When to get expert help
Get product-specific or professional help for anything involving floor loading, household electrics, unsafe plumbing, damaged glass, unexplained livestock deaths, or chemistry you can’t measure reliably. Use a qualified electrician for fixed wiring, and always follow the current safety instructions for your tank, equipment and additives. Never work around damaged cables, swollen batteries, leaking seals or equipment that keeps tripping the electrics.
Your livestock’s needs can also outweigh general equipment advice. Check adult size, behaviour, disease risk and each species’ light, flow and feeding needs with reputable care guides before you buy. This guide explains the equipment decision; it can’t guarantee the result for every animal or tank.
COMMON QUESTIONS
Frequently asked questions
How many watts of light does my reef need?
There’s no reliable rule. Your tank’s shape, the light’s spectrum and optics, how it’s mounted and what you keep decide.
Can I measure PAR with a phone app?
A phone can help compare changes, but it isn’t a substitute for a proper calibrated PAR meter.
Does blue light count for corals?
Yes, corals use a range of wavelengths that includes blue. Judge a light by its usable output and spectrum, not how bright it looks to you.