Refugiums are added for extra biological filtration. They are added to bring down nitrates and phosphates naturally, but they aren't the most affective way. If you have a fish that requires pods
to live (Mandarin Goby, etc.) then they grow in the refugium. Honestly, I think refugiums are too much work for such a little benefit. They aren't required at all. A more affective way to remove nitrates and phosphates is with a GFO
Rector, Carbon Reactor, etc.