A refugium is simply a 2ndary tank (or tub of water) that is attatched to the main tank. It will either contain algae or a haven for pods
The algae is either used for feeding grazers or filtering the water. The newer research is showing that it takes ALOT of macro algae to filter even a 55gal tank.
It can also be a breeding ground for pods
, worms and other benificial critters.
Now...why would you need all of those pods
and worms and other little creepy crawlies? Well...alot of fish eat pods
, such as Mandarin Gobies. And they are also very benificial for the over all filtering capabilities of the rock and DSB
If you have the room they are almost always a positve addition to a tank.