The pH of 1.0 reduction method, using the pH of "degassed" water vs. the water in the tank, only works if you know the ppm of CO2 in the "degassed" water. A 1.0 reduction of pH means there is 10 times as much CO2 in the tank water as in the degassed water. But, a 1.3 reduction means 20 times as much CO2. Big difference! And, the ppm of CO2 in degassed water can be between .5 and 3.0, depending on how long you wait and other factors unknown to me. So, this method is a comforting method, but far from an accurate one.