for a 21" deep tank I would go with 175w bulbs, if you have 250w bulbs it's good for clams but unless you have lots of caves it will be too bright for a lot of the lps corals.
!75w will give you enough light for clams and sps at the top of the tank
Here is a picture of my old 75g, it was 20" deep and lit with 2 175w 10k xm bulbs
