Consider this: If your nitrates are 50 ppm and you change 10% of your water, then you have decreased your nitrate concentration by 10% wich is 5ppm, ie your new measurement should read 45 ppm. If you change another 10% of your water the next reading will be 45-4.5=40.5ppm, and so on. Therefore, it is not as easy as it seems to lower your nitrate concentration by small partial water changes. It is better to make a major change for a short term solution, but then you must find what causes the problem and solve it (which is not the easiest thing to do by the way)