Many people "hook" their propagation tank with their display tank, including the store I work at. This way the corals do not need to be acclimated between tanks and frag processes, also it will provide stability in the water quality and lastly you won't need to purchase a whole other skimmer, heater, chiller,...etc. Typical set up is to have the overflow of the main tank drain into the propagation tank first, and then drain to the sump from propagation tank. Another set up is to share a sump with the display tank.