maybe I can help others. I have been helped enormously by this forum
I occasionally have had very low, off the chart, pH readings when using Taylor kit. I would do a second test and the reading would be normal, where I expected it.
The reason, I discovered, was that in the cases where I did a FC test first, I was rinsing the test tube but not the black rubber lid. The liquid residue from the chlorine test on the lid was enough to violently throw off the pH reading which followed next
the obvious: clean both the plastic testing vial AND the lid