I have a 60 liters (15 US gallons) tank. The height is 12". The open top tank came with a 15 W fluorescent tube that looks very dim (no real reflector, only an alu case). I added a 11 W lamp similar to those Arc Pods that looks brighter. This lamp is fixed above my zoanthids that are on a piece of live rock sitting on the sand bed.
The timer of the 11 W lamps starts almost two hours later than the main 15 W lamp, but all polyps show up in the morning even before the 11 W spot lamp comes up.
I followed the rule of one pound live rock per one US gallon tank volume and I am happy with with only two powerheads and nothing else.
To the reflector:
What kind of lamp is it?
For fluorescent tubes there are aluminium reflectors in the reptile section of some pet shops from GBP 5 to 9. Our tortoise has such a thing.
Otherwise it can be difficult to get the right shape.