If you're using a grow box it's best to have fresh air sucked in via a vent/fan and another vent/fan to push the stale air out. (Optional carbon filter can be added to this if you want odour control) Your vent to bring air in should be located at the bottom of your grow box so that the fresh air can rise up and drift over your plants in the process. And in turn your vent to push the air out should be located near the top as air and heat rise. Fans are also optimal to refresh the air but depending on the size of it you may just want to keep it on a fairly low setting.