[JustOneMore20]

DIY CO2 won't hurt the fish.......unless you have a bunch of 2L bottle setups on the tank, you are fine. Even then, it probably wouldn't produce enough co2 to hurt them.

You might want to start adding in a little Flourish (the liquid) to the tank after water changes since RO water has no nutrients. You don't have any needy plants, but it won't hurt (the plants or the fish) to add in some micro nutrients.