With the below-the-tank refugium (what I have), your return pump will be sending the refugized (I think I'm going to copyright that term

) water back up to the display. Some say that the impeller in the pump kills the pods coming from the refugium, but some also say a good bit still gets through unharmed. If you think about it, the majority of the pods that will be growing in your fuge will be so tiny as to barely be visible to the naked eye.
As capn says, with the above-the-tank refugium, the refugized water can be gravity fed, so no need for a pump. I've read some definitions of the term "refugium" to mean that it is meant to be seen, and not hidden in a cabinet or fish room.
On the other hand, not many people can or want to put another tank above their display tank. I like mine concealed under the display tank.
Tell you what though - I think it's a lot of fun just looking at the stuff that's sprouting out of the refugium. My son said it best the other day when he said our refugium was more interesting to watch than our 29g freshwater tank ever was!
