Keep in mind that Hanna state the shelf life once opened is 3 months (6 absolutely max if kept in perfect conditions), and also that there are 25 tests in a bottle, not 30. You're supposed to toss that last 5 ml.
I’ve used bottles down to the last drop with zero issues. I’m one of those conspiracy nuts. They don’t want you to refrigerate bc then you have to buy more as it starts to go bad. LOL I do believe there is a quality issue with the reagents. For different lots to be so far off, I don’t like it. I don’t test everyday anymore so back to Salifert I go.