While caulerpa and chaetomorpha are the standard issue algae for most refugia, they are not the end all and be all of natural filtration. In fact, there are lots of other marine plants and algae that are just as suitable, though probably more specialized in how they absorb. Some good alternatives to caulerpa and chaetomorpha are red mangroves, various sea grasses, and even corals and clams. The idea of a refugium is to provide a safe place for algae to grow, protect juvenile or otherwise damaged fish and coral, and let populations of tiny inverts grow to the point that they are sustainable. Any of the plants, algae, or invertebrate should thrive best in a refugium, assuming of course that the tank is all in good working order. Clams and certain corals have also been known to absorb nitrate and/or phosphate, and because of that, we often see certain refugia loaded down with corals like pulsing Xenia.