In regard to the pH changing in not very much time, does that still ring true for the water you have given your plants?
Could this change push you out of the golden pH 6.5 for soil?
If so how long are we talking?
Nope, the soil and whats in it help control soil pH at all times. Soil self pH's! The thing is, you water and the ph drops around 1 whole point. As the soil dries out again. The pH rises back up to "normal". This a
natural swing in pH, that allows the plant effective nutrient up-take, as some nutrients are better available at higher or lower pH values.
While soil self pH's and it's not
required to pH your in- going fluids.....Many synthetic users find peace of mind by simply pHing in-going. I tell folks who are wary of not. To simply go ahead and pH all in-going. 6.5 for veg and with synthetics I like 6.7 in bloom.
Synthetics are more prone to pH problems in my book....Taking that precaution is simple, and inexpensive.
Two simple ways to avoid a pH problem in soil are:
Don't over feed
Don't over water
That help?