What is the best way to control the CO2 level in my grow room?

Whether you use a CO2 Tank & regulator or a CO2 generator you should use a monitor/controller. You DO NOT have to use a monitor/controller but not having a monitor/controller could be unsafe & unstable. Having a CO2 level above 1500 ppm is not beneficial to your plants & without the monitor/controller the CO2 level will more then likely go up and down and up and down.