This site is a great place to get some ideas for sumps and refugiums.
http://www.melevsreef.com/links.html

A refugium can be made in many different ways. An HOB filter can be turned into a refugium. The area in a sump used for bio balls can be converted into a refugium. My secondary refugium is just a 29g tank on a stand next to the display that is supplied water via a Maxijet 1200 from the 55g sump under my 75g and it drains back into the sump.

A refugium is just a tank or container where you grow something that is safe from predators of what you want to grow.

Also, Xenia is not for everyone. For some people/tanks, it grows like wildfire and others can't grow it at all.

I think I know why. Xenia has been known to grow better in tanks with higher nutrient levels.When I was growing Chaeto or Caulerpa, I was feeding lightly like every other day and small ammounts and nitrates were hovering around 5.

I set up the second refugium, took the macroalgae out of the refugium built into the sump and introduced Red sea Pulsing Xenia and Silver Elongata Xenia and within 2 weeks, the Xenias were growing nicely and the nitrates dropped to undetectable levels.

I got to thinking....... maybe I could feed more and the SPS could benefit from the extra fish poop so I started feeding everyday and the nitrates remained undetectable. A nice side effect of more food and 0 nitrates is the SPS has better polyp extension, color and is growing and encrusting faster.

Myself and a few others have had very good results with Xenia as a nutrient export but I must concede that macroalgae has a very long and proven track record for this purpose where Xenia is still the new and much less documented thing in this area. Macroalgae requires light to grow and can do so nicely with just a cheap flourescent light from Home Depot. Xenia, on the other hand, needs better lighting. It can live and grow under PC's but does much better under T5's or VHO's and does a ton better under halides.

I currently have just 96w of T5HO's over the Xenia and am considering putting a 175 Halide over it to push the growth faster.