You should NOT store your Hanna meter in regular calibration fluid. I bought in to the idea that you could do so because it says so on the GH calibration fluid bottle, and I did so for about a year. One day I looked closey at the probe and electrode and they were all scummy. I got a bottle of Hanna Electrode Storage solution and that what I now use. As someone said, this solution had a PH of approximately 4, not 7 as the calibration fluid. This is more acidic and has a definite cleaning action on the electrode and probe. My advise is to use the Hanna solution, but hey, whatever works for you