CO2

If you're using fluorescent grow lights like CFLs or T5s, there's probably already plenty of CO2 in the air for your plants to process all the light they're being given!

Things are different when there's more light being provided than your plants can naturally use. In high-light situations, supplying more CO2 can allows your plants to use more of that light, which results in increased photosynthesis. By getting your plants to produce more energy from the same amount of light, they'll reward you with faster growth and bigger yields! As well as an increased resistance to light and heat stress!