Simply aging tap water can actually lower your nitrates. Fill a bucket with tap water. Take a reading. Then out an air stone in it for 24 hours. Re test. 80% of the time you will get a completely different reading. However, say your tap has 15ppm nitrate, but you have enough plants to consume 20ppm nitrate, then you're short 5ppm if you do not dose. But....if your tap has 25ppm and your plants consume 10 ppm and then you dose 20ppm then your nitrates are too high.....but honestly.....plants really don't eat as much nitrate as people think/wish they do.