Assuming the fish health as a priority over the plant and algae situation?
When starting the CO2, it may take a bit for the fish to get used to the new water situation. That means start at a low level and increase it gradually as you watch for the fish to act differently. Watch for less activity or less interest in eating. If you see signs of this, they may be having a bit of trouble so let hem get used to the current before adding more/higher levels of CO2. If you feel they are really looking bad, do a quick water change to get the levels of CO2 down. Depends on the fish how much they will react but I find most will adapt quite well but they do need time! Somewhat like us going to high altitude, it is better if we can do it over several days or even weeks for some. Early morning seems to be the time when my fish react to cO2 the worst so watching and really, really asking yourself if they act weird is good at that time of day.
Ferts seem to be far less trouble for them. But then it depends on the fish there also.
Watch carefully, don't do changes when you will not be there to watch and take action before it gets really bad. Keeping a plan for quick water changes is a good safety move.
The old saw? Proper prior planning prevents poor performance? Goes double for fishkeeping!