If you are using a lower heat light source like T5s or LEDs, and have an A/C unit, I would suggest running a closed room. The co2 will replenish what the plants need so no need to bring in outside air and suck out all your co2 in the process. co2 isn't needed at night, only when the lights are on. If you do need to exhaust I would suggest looking at an environmental controller. There are some pretty sophisticated ones available that will control not only your co2, but also your exhaust fan, a/c unit, and humidifier/dehumidifier. Unfortunately they are pretty expensive.
I would also suggest you look at a co2 burner over using bottles. The up front cost is a bit more but they are a lot cheaper to run. In a tent that size one 20 lb. propane tank should last for at keast 2-3 grows. I paid $250 for my burner and $300 for the controller.