several replies coming in are all saying the same thing, your nitrate production is most likely much higher than 20, it is the hair algae feeding off of it that is keeping it at that level.
as for what is producing all the trates, that is another story, but from other boards at least, the consensus so far is that if you drop your trates, the algae should start to go away. if you can not find an immediate source of the trates, then definately a nitrate sponge could be in order. also, do you have a good population of macro algae in your refugium? if not, adding some more will help naturally absorb some of the extra trates.
another reoccuring thought is that you might want to cut back your feeding to every other day for a week and see if that helps anything at all. your fish shouldnt need to eat constantly unless you have anthias or something, and skipping every other day will help lower the waste and thus nitrate introduction significantly.