20-30ppm is a reasonable, abtainable, goal with w/c every other week. Although if you have a monster you will most likely need to go with weekly w/c. With my discus tank the nitrates are never above 10 ppm, My aro is kept between 10-20ppm the rest of my tanks may get as high as 40 ppm but i am adjusting w/c in order to lower that. By the way my planted 72g the nitrates are at zero.