The drop in pH caused by CO2 dosing shouldn't have any harmful effects on the fish. And like I said in my previous post, the only somewhat accurate way that the typical hobbyist can measure CO2 is by using a drop checker with 4dKH solution.
+1. A drop checker is the cheapest and most convenient method of measuring ("estimating") the CO2 level in your aquarium.
There are more expensive options, but these are usually outside the price range of the hobbyist.