CO2 Concentrations

        I have got a 150 litre ~(40 gallon) aquariun. It contains a home
made yeast CO2 system. I am currently dissolving 30-40 bubbles of CO2 a
minute into the tank.
        When I used my CO2 test kit it showed a concentration of 100 p.p.m.
Could this possibly be right? In texts I have read that the recommended
level is 25-35 p.p.m. The fish are not showing any signs of stress. Could it
be to do with the fast that my water is VERY hard? Also before using CO2 the
pH was >7.8 and now it is 7.0.