you can get below 20 ppm nitrate readings my odd ball tank's nitrate reading is below 0.5 ppm I do a 30% water change every 2 weeks and is quite heavily stocked but I have a lot of floating plants that suck up the nitrates. My channa pulchra tank I change 20% water every 6 months getting about 0.5 ppm nitrate but then again the tank is very heavily planted.