Typically when using a PH probe to measure your tank PH you Calibrate at 7 and 10. If you are using the probe in a CA reactor you Calibrate and 4 and 7.
The difference is very noticable when using the probe in the reactor. I went from calibrating at 7 and 10 to 4 and 7 and my probe rose 0.4. That is a lot when it come to melting Calcium carbonate
J
|