I doubt it's a test kit issue if your using quality kits. Titration method testing is more accurate for Ca, Alk and Mag then the digital colorimeter tests. Info direct from Lamotte technical support.
My suspicion is it could be related to carbon dioxide levels in the house and consequently in the tank. How does your pH fluctuate during a day? Monitor it over a week long period, say every 3 or 4 hours. There may be a correlation between co2 levels in the tank(lower pH) and the alk issue. ie High co2 would cause the alk to be depleted in the tank.
Cheers,
Tim
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