The idea that the rock will reduce the nitrate on its own is correct. Anaerobic bacteria will convert the nitrate to nitrogen gas and that will bubble out (not usually seen). If those bacteria aren't there or the rock is not suitable for the anaerobic bacteria this won't happen. It could take several months for the anaerobic bacteria to colonize in population significantly enough to consume the nitrate.
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~ Mindy
SPS fanatic.
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