Honestly, look at buying the right lights the first time and not the second. 250w on 18 inches of depth? Your corals will love you, but you need to make sure that whatever corals that you do get are acclimated to the light over a few days so they don't bleach! Make a nice TALL canopy, that way you can move around, add rocks etc without having to remove the canopy. I am sure you will hear stories of people burning an arm or two from not having sufficient room.