DIY or dry rock can save you money nut 1. you have to get it right 2. you need a cerain amount of live rock to make sure you get enough variety of filter bacteria. The bottled cultures contain only on or two species, of any, and there a lot more involved than are commonly thought. However I use some of the premade structures from aquaroche to put real live rock on as they are premade large towers and caves from effectively moulded limestone.