CO2 shouldnt bother the fish in the slightest as long as its at or under the 30ppm threshhold. It alters pH but nothing else related to the water chemistry, when people turn it off for the night, pH in planted tanks usually spikes.
make sure you get a drop checker AT LEAST if you are going with any form of pressurized CO2 so you can actually adjust it properly
and finally, CO2 only has benefits when used during the day with the tank lights on as thats when the plants take it in. At the moment, the CO2 you are currently using seems very far from the maximum, if it helps the plants, great!
air driven filters and CO2 dont mix however... you are outgassing the CO2 about as fast as it goes in like that.