Thats interesting about GetThisOrDie's well water not doing it.

Our tap water comes from underground, treated and I think mixed with other recycled treated water. By the time it gets to us its a steady PH 8.
Maybe I should try leaving the water to outgas for another couple of days and see if that makes a difference. Its worthwhile trying a few things as it would be handier to be able to mix the water for a few days in advance.

Thanks everyone for your input, much appreciated