nitrates are driving me up the wall

[68shelby]

Any idea on how to get my nitrates down, 100g tank. Livestock consists of 1 yellow tang 4", 1 regal tang 3 1/2" ,2 clowns, 2 chromis, and 5 blue devils, 2 shrimp, 1 lobster, assort crabs, and 4-5 snails. all livestock other than the damsels are 1-5 years old. I only use RO water. My nitrates are consistently between 10-20 ppm. Regular water changes 160 gallons in 2 months. I have no filter media, skimmer is skimming the best it ever has. I have no hair algae but the brown and purple diatoms on the sand are constantly there. And I do not overfeed, however there main staple is mysis shrimp, which I rinse first and drain off. Once a day, and once a day Tangs get sea veggies, (other fish eat it too. As for the corals they get reef roids once a week. HELP PLEEEAAASSSSEEE ,,