After measuring KH, I would start by figuring out why your 7.6 tap turns into 6.6 tank water. That is a dramatic difference. (btw, try high range Ph test on your tap, it is probably higher then 7.6).
The usual suspects would be excessive co2 injection or something really rotting, like driftwood. Or a bottle of vinegar.
The probability of your tap going more acidic by just sitting in a bucket is close to none.