Ulva or sea lettuce is not only good at filtering nitrates from your tank, it is an excellent food source for tangs and other herbivorous fish. And if you get hungry enough, it doesn't taste that bad either. :)
This green macroalgae is very easy to care for, and will do fine under a wide range of light, but does best under bright, full spectrum lighting. It can be attached to shells or liverock, and looks great in a macro display tank. Amphipods will appreciate this addition to the refugium, as they will snack on it from time to time. It is a fairly fast grower, and is considered exceptionally hardy.