Doing a water change with water which contains Nitrate will not necessarily increase Nitrate in the tank. If I do a water change on a tank which reads at 20 Nitrate with tap water which contains a Nitrate level of 5, then some change will happen -- it just won't be as drastic as if the tap water was at a level of 0. I'm sure there's math to go with this, but I don't do math, and the concept seems easy enough to me -- If I have strong kool-aid and remove some and add some really weak kool-aid, the remaining kool-aid will still be stronger-tasting than if I'd just added water, but not as strong as it was before.