I'm going to buy atleast 30-40kgs.
From memory I think I put 20kgs in my current tank and its not enough for say the elegance coral the clam etc.. Over time some of the sand disappears or gets pushed to the back of the tank due to the water movement etc etc..
My elegance is huge (and I mean huge!!), with a big base at the bottom. So I need atleast 5-6cms of depth for it.