The drop of 1 in pH means the concentration of CO2 increased by a factor of 10. Since you don't know what the starting concentration was, you don't know what the final concentration is. The assumption that the starting concentration was 3 ppm is behind the assumption that you have good CO2, 30 ppm, with the drop of 1 in pH. But, the starting concentration could have been 2 or 4 ppm, or something else.
You can increase the concentration of CO2 up to the point where the fish start clustering at the surface "gulping air", or fish start laying on the substrate or losing their colors. Those are signs that you have too much CO2.