I have a Hanna Checker PH meter and in the 7.0 and 4.01 cal. solutions the meter reads what it's supposed to read, but once I put it in the water from my res. it reads ~6.15 no matter how much ph down I use. Once I got to my usual dose of down I stopped adding because I didn't want to crash the PH too low. I just ordered a new meter anyways because I don't feel comfy with this one any more.(not the first prob.) Cost $20, guess you get what you pay for but I thought Hanna was a trusted name.