Can I buy and use just the fuge rack or do I need the media rack to hold it in place/get water to it?

http://www.nano-reef.com/forums/index.php?showtopic=216277

That ^

And yes you can use it by itself. It is not supported by the media rack at all in function or stability.

Here is a video to help you see that

http://www.youtube.com/watch?v=7lMfaYMidV4