Time to make a point! The water entering the refugium should come from below the main tank's water surface not from the overflow, if so equipped. Why? Surface water is skimmed from the main tank into the overflow. This surface water is laden with oils and other dissolved organics. Why would you want to dump this into your refugium? Better to place a small powerhead inside and just below the water surface of the display tank to provide oxygenated H²O! Better yet is to tap off the return line from your protein skimmer! Highly stripped and highly oxygenated water from the skimmer provides as near ideal water as we can expect.
Draining the refugium back to the sump allows gravity to do the labor for us. If your system is sump-free, then mounting your refugium above the main tank waterline or behind the tank may be desirable.
Ok, we have established what a refugium is and how it should be connected to the balance of your existing system. We have seen why it's important to consider the right plumbing and to plan accordingly. Now let's discuss what to place inside, preparatory to introducing our animal load.