I'm having a hard time understanding nitrate levels in a planted tank. Currently, I'm dosing E.I. fertilizer and my nitrates levels appear to be in the 20 ppm range (sometimes I second guess the color matching test kits and not sure what I have). I have also noticed that EI ferts. contain nitrates. I have also noticed after reading some of the post that a lot of people use Seachem Purigen in there filters. Wouldn't this remove all of the nitrates from your tank? Doesn't this defeat the purpose of adding fertilizers?