Base rock is large pieces of live rock. What sand do you have in your tank? did you buy "live" sand from a LFS or did you purchase it from an established tank? I would purchase as much purple rock as possible and haggle your buns off. Even if you don't have room for all of the rock you buy, buy it. You can do wonders in a 75 with alot of rock and sand. I barely fill my cup in my Euro Reef RS135 weekly because the natural filtration works so well. I don't even use my skimmer on my 48....