For those with low pH, and if your alkalinity readings are good, then you most likely have a problem with the air exchange at the waters surface.
Either there is not enough turbulance at the surface or, the more likely cause is too much CO2 in the household air.
To see if this is the problem, take a sample of water and check the pH.
Then, aerate the sample outdoors for about an hour or two.
Recheck the pH, and if the pH is now higher even by a little bit in that short time, then you have a CO2 problem.
This problem is common in households that are shut up for winter heating or for summer air conditioning.
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