Your diatoms and nitrates will go diwn in time if you do regular water changes and maintanance. You can still have corals with 20ppm just better for less. To get tozero and stay there you may need to take extra steps , take a look at the nitrate poll thread which was posted last week. My little tank has never hit zero I still have it fully stocked and everything does fine minus a few picky acro's lol
