In order to get a proper pH test, the water needs to be tested at the time of sampling. CO2 can either be absorbed into the water and lower pH, or be lost from the sample and raise pH. Temperature changes and biological processes in transit can change pH dramatically in low buffered waters.
To get an accurate pH, test when you sample, and take several small water samples and make a larger composite that will more accurately represent the pond as a whole.
Mike
