Carbon dosing (sugar etc) and aggressive skimming
are most often used on sps tanks to achieve the low nutrient levels required by these corals. It is the basis for Zeovit, Prodibio etc. Water changes are to balance the elements in your water that may get depleted with time. Doing water changes to reduce nitrates is expensive, time consuming, and a loosing battle IMO. Better to buy a bigger skimmer!