The answer made here about the aquarium co2 is my favorite too. Google the instructions, it’s A simple 2cup sugar, 1/2 tsp regular baking yeast and 1tsp regular baking soda recipe for a 2L bottle that needs some extra steps for activation, once it’s going you connect some airline hose and use a little airline tap/Fawcett to control how many bubbles per second are being produced (place the tip of the hose in a glass of water to check this). Then place the tip of the air hose near a fan in your grow to disperse the co2. Make sure to take it out at night or place it outside where co2 can’t contaminate your home or over suffocate your grow during lights out. It’s nowhere near what a co2 bottle and regulator can do but it will definitely enrich the air a bit more with co2.