Is supplemental CO2 absolutely necessary with continuous air flow?

My current set up is a 3'x3' tent with a 600 watt HID and 6" inline fan. I have my inline fan running the whole time my HID is on because of temperature issues, and for new air circulation I use passive air intake through an open window. New air from outside is constantly flowing in, but I was wondering if this would be enough CO2 to sustain my babies throughout veg. and flower, or if indeed I would still need supplemental CO2.

Should be fine. CO2 is more beneficial in sealed rooms.

I run in my basement. I exhaust all my air back into the basement, and get all my fresh air from the same basement. no open windows, ect in basement. kind of recirculating the air down there. I do have one A/C vent open for some fresh cool air. almost 2 moths growing and no issues. knock on wood.

not at all necessary. u do need good air turnover, i have nothing special and grow in a bedroom i keep the door cracked at sleep time and open when im home during theyre daytime. i did experiment with yeast/sugar generator for a little. by no means worth the trouble for me.

CO2 is for sealed rooms that have been dialed in. As long as there is plenty of fresh air you'll do just fine with that setup.