When adding new rock to an established aquarium, do so slowly and one at a time. A mature reef tank is a carefully balanced ecosystem. When you add a new piece of live rock, or any livestock, that balance is disrupted. The larger the disruption (e.g. a bunch of fish at once) the farther off center that balance becomes. This could potentially lead to livestock losses and a bloom of nuisance algae. For these reasons, changes to an aquarium should be small and spread out over a great deal of time. When adding live rock to an existing tank for example, I wouldn’t do more than a single large rock per couple of weeks, even if the rock has been pre-cycled.