Two keys about KH in your tanks.
Your KH needs to be at least 3 in any tank you are injecting CO2 in. Anything lower and you will have large swings in the pH that can stress your fish and kill them. Too high and your CO2 levels will kill the fish. 3-8 is a general safety range.
As for target levels for pH, this depends on your KH. CO2 Calculator
. Download the calculator and play around with it. The key thing to remember is you want to have between 30 and 50ppm of CO2 and that you keep it steady. Steady levels is more important than high levels because fluctuating levels will cause algae, this is why I don't inject anymore. If you want to play it safe to start with, stick with the levels recommended by the calculator. It will grow plants and will be less likely to have your fish stressed or gasping for air.
Figure out what your KH is then figure out where you pH needs to be for both safety and high enough levels for best plant growth and that is what you go.
And to reiterate what Jasyn said, make sure your fish are going to tolerate the KH levels you have before messing with it.