Do not try to get the Nitrates to zero. For some people its pretty much impossible, besides unless u have SPS that require perfect water, 5-20 nitrates is not bad, but do not go over the 20, never. IMO lots of people don't want to admit the truth when they say that their nitrates are at 0, or they just starve their poor fish to death. I have a large refugium, completly over grown with chaeto and still, could not get it lower than 10. Feed my fish one cube of mysis a day, its gone in 10 seconds, and without water changes I am at 20.My corals are growing, inverts are very active, fish are insanely active, so I would not stress, or you will develop an obsession. Besides lots of softies like 'dirty water'. Keep an eye on the phosphates when it comes to algea, and corals don't like it either. Make sure you have enough circulation.