My little drop checker is working great now. I have adjusted the bubble rate a few times and am now keeping the checker color on green from about an hour after the CO2 comes on until it goes off. Here is a photo of it in the tank - green!



I see no signs at all of the fish being stressed by the CO2. So, my next step will be as Neil suggested - change the indicator solution to KH=5, which will make it be green at 40 ppm. Then I will watch to see if the fish have any problems at that ppm. If not, and after a week or so, I will try KH=6, which gives 45 ppm when the color is green. I keep wondering if we reallly know what the maximum CO2 concentration can be without bothering the fish, since I don't know how anyone knew what their concentration was using the methods we had to measure it. And, is there any theoretical way to determine what is too much for the fish? I do know that 70 ppm+ is too much because I had that briefly when I first started using this device and all of the fish were gulping air at the top.