Earth has 300 ppm for free , just use lots of it. NOW CO2 TANKS MUST HAVE A MONITOR , OR IT CAN KILL YOU. REMEMEBER NEVER GO AFTER A DEAD MAN. . NOTE YOU CAN NOT SMELL OR TASTE CO2. Open all doors and vent the space before going in. Note about 50 people die each year for co2. The 1000 bucks for a monitoring system is well worth it. You must keep the background ppm to 1500 for max production and keep the heat up to about 90 df. to maximize you output.
Please do not kill you self over a little pot. Although coffins are much more expensive than co2 monitor. save money and live