Good job VaughnH, and good thinking. It will be interesting to hear how you get on with this.

A few things occurred to me while reading you post:

I’m no expert on ph indicator solutions and don’t know how much they vary but it would perhaps be useful for anyone using this method, that has the opportunity, to use a well calibrated ph meter to check at what ph their solution becomes a particular colour. If anyone does this they could post their findings for the benefit of others.

A lot of people including my self set their maximum CO2 level (bubble rate) by careful observation of the fish. In my case I increase the CO2 slowly and carefully until the fish start showing signs of breathing faster/more heavily and then back it off a fraction so that the fish are breathing normally. Using this method and based on the ph/kh tables (using a well calibrated ph meter) my tank apparently runs at around 90ppm. I think many people using the kh/ph tables are finding that they apparently have even higher levels of CO2. While many fish can almost certainly tolerate considerable more than the often quoted 30ppm it seems likely that as VaughnH suggests the very high levels many are apparently getting is due to the effect of other acids in the water.

What I think would be interesting would be to try and establish roughly what the CO2 level is in tanks where the level is set as high as possible using observation of the fish. A possible solution here based on VaughnH’s idea would be to establish at exactly what ph the indicator becomes green. Then use a range of solutions at different kh values (e.g. 4, 5, 6 etc) prepared as suggested by VaughnH to establish what kh is needed to achieve a known ph in a tank with maximum amounts of CO2 added based on fish health and consequently the CO2 level in such a tank.

Cheers
Neil