Help, cant get nitrates down in cycled tank?

So, 75 gallon has 6 large live rocks and 3 small one. Ammonia and nitrites are measuring zero. I have a zebra turbo snail, 3 margaritas, 2 cerith snails, 2 emerald crabs, and 2 peppermint shrimp. They have been fine the past week and are very active on the algea-covered rocks. However, try as I might, I cant get the nitrates down. a 50% water change brought it from 40 ppm to just under 20 ppm, but no further. Any ideas? Would adding more live rock help? We plan to use smaller live rocks to build a "corner", for a pair of clownfish I plan to buy on Thursday. In about 6 months time I want to add an anemone. How do I get them down? And will the clowns tolerate 20ppm? My 5 year old is extremely impatient. We also plan to add mushroom corals and a coral beauty over a period of a few weeks. Any advice is appreciated thank you.