the micro/macro algaes that you can grow in a refugium will thrive on disolved organices other thrive on phosphates and nitrates but in small amounts. there are fauna that will eventually begin to develope that will consume the nitrates, these fauna live in the parts of the system that are low in oxygen, deep in the sand bed.
a refugium is basically a refuge for organisms that can't live in the main display tank do to abusive tank mates or injury. also a refugium can be a place in which you can grow pods
that can be used to feed so spieces of fish