Co2 Levels

I have a feeling that this subject has been covered before but I can't find
it anywhere.

The optimum Aquarium lists CO2 levels as a function of KH and pH.  In my tank
with a pH of 6.8 and a KH of 4.48 (80ppm x .056) I should have a co2 level of
~18mg/l which I assume is the same as ppm.

What are the adverse levels. I am now reading 45ppm and have read as low as
5ppm. When I saw that the level had risen to 15ppm, I disconnected my yeast
co2 bottle overnight. The next morning I reconnected it and after about 2hrs
I got the 45ppm reading.

I also assume that at some level the co2 will start effecting the fish. Any
guidelines anyone.