I use Excel too, but as a supplement. I have high light and a CO2 system. One really has little to do with the other.
Actually, while you may call it rescuing, you just added to the problem yourself. It is impossible to rescue them all - the more that get rescued the
Change the rainbows to tetras or danios, and get rid of the butterfly fish.
What are the dimensions of your tank? The African butterfly fish would probably be too large for a 20G tank.
With that said, can you still dose excel if you are injecting co2? I'm not sure what reason you would want to do that for. I'm just debating on a co2