a refugium is a special "safehouse" for plants to grow in a marine tank, these are normally placed in a chamber in a sump but they are also found in HOB designs as well as in tank ones. Basically its a chamber filled with marine macro algae. The algae is under a light normally in reverse light cycle (when the tank lights are off, the refugium lights are on, vise versa) and this light grows the plant. THe reverse light cycle helps stabalize pH during the night, and the plant itself absorbs phosphates and nitrates and stuff. In addition to this, since the fuge has no predators therefor large population of pods then take the fuge has a home, breeding uncontrollably, so much that there isnt room for all of the pods, with that, the "extra" lose their grip in their home and go shooting up the return pump into the tank where it gets eaten.