I keep tap water treated with stress coat and florapride in gallon jugs, ready for use... just tested that for ph, and it was similar at around 6.4
Are you saying that your water directly from the tap is 7, and it is 6.4 after sitting but before CO2? If this is the case, then 6.4 is the natural pH of your tap water and your DIY CO2 is ineffective.