Subject: Determining CO2 in H2O soln using carbonate hardness and pH

Tropical fish hobbyist add CO2 to the aquarium water in an attempt to 
enhance aquatic plant growth. The goal is a CO2 range of 15 to 20ppm. This 
range results in improved growth of aquatic plants while remaining safe 
for the fish. Aquarist attempt to determine the [CO2] using measured pH 
and carbonate hardness (KH). Rather than blindly accepting the formula 
given by fellow aquarist,([CO2]=3*KH*10^(7-pH), I would like to understand 
how it is derived. As of yet I have not received a clear explanation. I 
believe the starting point is the Henderson-Hassalbalch equation pH=pKa+log
(proton acceptor/proton donor). The assumption is that carbonate is the 
only buffer system and temp 25C. Also some people feel that adding CO2 to 
low KH water can result in rapid/large pH swings, is this accurate?

Re: Determining CO2 in H2O soln using carbonate hardness and pH