The five-step method
Record internal length, front-to-back width and water depth in centimetres—not nominal tank litres.
Mark where coral will actually sit. A central island needs a different light field from wall-to-wall rockwork.
Compare the maker’s recommended spread, mounting height and depth—not the largest “up to” number alone.
Branching coral and tall rockwork increase self-shading. A second source may improve geometry even when one light has enough power.
Add mounts, rails, controllers and realistic electricity. Confirm UK warranty and replacement-part routes.
Begin conservatively, use acclimation controls, observe livestock response and obtain a PAR reading when coral demands justify it.
Planning bands—not prescriptions
| Tank footprint | Starting layout | Fixture class | Main caution |
|---|---|---|---|
| 40–45cm cube | One compact fixture is the normal starting point | AI Prime 16HD or ReefLED G2 60 class | Rockwork overhangs and edge placement can still create shade |
| 50–60cm cube | One broader fixture may work; verify depth and coral plan | Reef Flare Pro S, Kessil A360XE or XR15 class | A headline 60cm spread is not automatically uniform SPS coverage |
| 90cm long tank | Plan two compact fixtures or one proven elongated system | Two-point layout gives placement flexibility | Check centre overlap and dim edges before buying |
| 120cm long tank | Usually two larger fixtures or a multi-light rail | Budget the complete mount and cable route | One high-power puck rarely gives even end-to-end coverage |
Why depth changes the answer
Water depth increases the distance between fixture and target. But simply choosing more watts can worsen hotspots at the surface. Use the manufacturer’s actual test geometry where available: Red Sea, for example, publishes fixture height, tank dimensions and readings for its G2 series. That is more useful than a bare “suitable for 200 litres” label.
The deliberate uncertainty
Water clarity, surface movement, mounting height, lens choice, spectrum, intensity and rockwork all affect delivered light. This is why Reef Bench labels coverage as a claim and does not translate watts into PAR. Different fixtures can draw the same power yet distribute it very differently.