Banderbe,

Here are the facts as I see them:

Untreated, your water is GH 22 KH 18, which is very hard. You inject 3 bubbles per second of CO2, and your pH goes to 6.8, which yields a CO2 level of 85ppm. If you back off to Tom Barr's suggestion of one bubble every three seconds of CO2, your pH goes to 7.2, which yields a CO2 level of 34ppm.

What is the problem? It sounds like if you stick with one bubble every three seconds, you have the perfect CO2 level (Tom Barr's recommendation was a good one, as usual.) Then start looking at the rest of your nutrients.