I would say your wasting space, a 90g is too deep for a clam tank(assuming the footprint is 48lx18w) I'd go with the 400watt MHx2 with 2ea 96watt Power compacts, if you have to use the 90g . Using those 175's, you'll have to set any corceas or maxima's so high up you won't really be using the bottom 90% of the tank. Just MO. Go big,so you won't have to re-buy later.