Plants need the co2, and produce oxygen. CO2 is the primary concern when it comes to bringing fresh air in, plants need it live. The normal air pulled into the chamber is going to contain 300ppm give or take of co2. Putting it to 1500ppm makes a huge difference in yield time, density, and bud size. Plants have a maximum potential that they can not exceed, but to get there, all requirements must be met.
Sealing that grow and using co2 would not be hard. You can use a smaller tank than most (5lb) and release it on a timer if your box is sealed (no controller needed, just some math) and a 5lb tank could be concealed pretty easily in a speaker or something small like that. You would need a regulator with solenoid though.
Very nice work though with your box. I give you plus rep for building it.
