02-07-2012, 07:45 PM
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Just out of curiosity, did you shake the second bottle of the nitrate test for at least 30 seconds if you are using a liquid test kit....check and make sure you test kit is not expired.
It is very possible but highly unlikely that with doing 25% water changes weekly then you might have zero nitrates. Could you perhaps skip a few extra days and see if a reading shows up? If it does then just do a slightly bigger change next time.
Each water change should reduce nitrate numbers by half.