Mike, my understanding (correct me if i'm wrong...which i'm sure i am), of sand replacement, is to keep some of the sand from clumping and holding onto the undesirables, such as phosphates. In my mind, it is similar to pruning macro algae...nutrient export. Get rid of it before it eventually leaches back into the system. Every so often introducing a small amount of fresh live sand with a detrivore kit of some kind...will keep the sand bed from exploding, like Rogue's picture....
