I don't think I can push any more CO2 into the tank!

I built one of your internal reactors and it works well, putting out a nice fine mist.
In the UK, we have access to a commercial CO2 test that works the same as your CO2 drop test indicators and it shows a nice light green / yellow indicating a high CO2 content. I also have a pH monitor which shows a pH of 6.1-6.5, depending on time of day, with a kH of around 130 mg/L (a Tetra kH drop test)
If I try and push a little more in and the pH drops to 6.0 or less then the fish start looking very stressed and head gasping for the surface

I can't think how I can improve it any. Nutrients you have noted before and seem fine.