Some may get sucked into the overflow, but it's usually the other way around. They live and breed in the refugium, and they typically get sucked into the return pump and deposited in the tank. They're tough, so most of them will survive the trip. I have a refugium, and I have to occasional re-seed it and the tank. I use a copepod and amphipod mix, and usually buy a couple thousand when I purchase them. The company I purchase them from frequently has a BOGO sale, so it's not too expensive to buy that many at a time. The pods I put in the tank are usually eaten fairly quickly, but the ones in the refugium have a chance to breed and grow without any predation. It's hard to estimate the population during the daytime, so I usually wait about 30 minutes after lights out and scan my tank. If I see a good many amphipods, I know I'm in pretty good shape. If I only see a few, or none at all, I know it's time to replenish them. It's been over a year since I bought any, so that's not too bad. I don't really have to replenish the copepods, as I can see them crawling on the glass in spots where the flow isn't too strong (around the corners), but it's cheaper to buy the mix than to buy amphipods alone. It all depends on how many fish you have eating them as to how well they will maintain their population. Long story short... a refugium will definitely help with keeping a steady population of pods... but you may have to supplement occasionally.
