I am told that the contoller is new and was calibrated with 7.01 and 4.01 solutions... Just a little surprised that given the parameters, a change of -0.1 was the difference between fine and dead. I am thinking there must be more to the story.

I run my CO2 very high in one tank - not sure but must be at least 40ppm - straight up yellow in my drop checker with 4dkh solution.... the tank has been thriving for more than a year.