If the test kit reads 7.3 and the controller reads 6.1, one has to be wrong. The problems with test kits are in judging the color of the sample versus the little color chart, but 7.3 looks much, much different from 6.1. So, I doubt that the controller is reading correctly. I don't use one, but I know you have to do a two point calibration for it to be accurate. That would mean having a test solution with PH=7.0 and one with PH=6.0 or 5.0. If you only have the 7.0 solution that could be the problem. In any case even 6.1 isn't a low enough reading for panic. If the fish are all at the top of the tank gulping air, you have too much CO2. The fix for that is to reduce or stop adding CO2 until the fish act normally. If the fish act normally you don't have too much CO2.