The rock and DSB are optional, but as long as you don't fill the fuge with more than 1" of livesand you will be fine no ammonia spike at all. More than that and it is possible to see a small ammonia spike depending on your tank volume and waste processing ability. I would use dry sand to whatever depth you choose and then add a pound or two of live sand, make sure the live sand is sold out of an aquarium not a bag.