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
So far I've be planning what I want for my 21 gallon for an extremely long time. I thought I wanted a nano fish tank, then a shell dweller tank, and last