Just did a water test, 1 week ago my Nitrate (NO 3)ppm (mg/L) was in the safe range at 0. Now it is at 20, still in the safe range on my Mardel 5 in 1 test kit. The other nitrate (NO 2) is showing 0, when i look at the dip stick the test strip is still white indicating 0. Are these ok, I just did a 25% water change yesterday.