A little more info on your setup would help. However, some general info on co2 might help.
You'll see the biggest gains in soil, since hydro is already helping the plant grow at full potential.. some say 30% shorter lifespan and 30% bigger yield, but i cant speak from experience. I've only gone through one crop with co2. You don't necessarily need a monitor. You can get a regulator and just set the flow according to the size of your room. Im not sure of the formula to hit 1500ppm but im sure you can google it and it will come up no problem. But if you do decide to go with the sure-fire way to know your girls are getting optimal co2, i suggest the CAP PPM-4.
With co2 your plants can also thrive in higher temperatures, and prefer 80-85. I keep my temps at 83. Youll also want to stop supplementing co2 within the last couple of weeks of the plants life while its working on thc production.