tested the well water, nothing different as far as I'm concerned
Hello; Let me be more clear. By testing the well I was thinking of some sort of professional or government water testing facility.
I follow that you are doing your own testing. If you are new to testing there can be procedure mistakes which give false readings. My experience suggests the use of a standard for the pH meter and the other water parameters. Do you have distilled water available? If I am thinking correctly distilled water should have a nearly neutral pH and should test at or near zero for ammonia, nitrite and nitrate.
Distilled water is also useful in rinsing out the tubes used during water testing. Perhaps three rinses of the tubes between each test.
Back to my suggestion of 20%-25% WC. This may reduce the shock of the differences between the tank water and the new well water but is not necessarily a solution.
A possibly faulty assumption is the well water is more likely to be stable over time. Using this possibly faulty assumption and also assuming you water testing is accurate, it would follow there is something going on to alter the tank water pH so much in a week.
A last thought. People are known to poison tanks. I have had it happen to my own tanks. Know of other folks having the problem. Some posts on this forum.
Another issue is the tank equipment can become contaminated. I have buckets used only for my tanks and different buckets for other chores. I mark each type so I don't mix them up. Is it possible other people have access to your tank equipment (not just buckets).
