it depends on how fast or how much No3 u want to reduce. if your no3 is 50, by doing a 50% water change, it should drop to 25, another 50% will drop it to 12.5, another 50% will drop it to 6.75, and so on. Ideally should be this way but depending on tank vol too. Those ppl with large tank, before no3 drop, either they drop or their wallet drop.