for 18” height, you don’t need a ton of power. And since most lights have a square coverage, the 18” depth will mean each puck style light should only cover ~18”/48” to avoid wasted light spill. In short, your issue will be getting enough light spread not necessarily getting lights to punch deep.
So you could do 3-4 puck style lights, 1-2 rectangular coverage lights, panel, or bars.
A few popular lights that come to mind in these categories:
Puck:
- AI Prime
- kessil 360x
- Noopsyche k7 v3
Rectangle:
- kessil ap9x
- radion xr30
- orphek icon
Panel:
- reefbreeders photon/meridian
Bars:
- quanta helix
- Orphek or3/or4
- AI blades
I personally am using t5 led hybrid with 1 noop k7v3 and an aquatic life t5 hybrid fixture over a 60g cube (24x24x24).
Im likely downsizing to a 40 breeder after a move soon. The plan is to use 3 noop k7v3 for a full sps tank. Noops are less polished than brands like kessil and AI, but par for $$, they are great value and grow corals.