Don't bother with Co2. You're growing one plant in a 3 x 3. There is no cost benefit. Co2 is best used by growers that have everything dialed. They have their growing down and are using a sealed room. I know you've probably read a bunch of stuff about the benefits of Co2 but it isn't going to make a difference growing one plant in a 3 x 3 tent. And will likely distract you from the tasks you need to focus on.