How much light are you running? You can over saturate if you don't have enough, have had leaves yellow and fall off, although 800ppm is low unless your meter is off,
Over saturate? Like say what? As long as the lights are properly spaced away from the canopy. Your not going to do what you say!
There is a light saturation point in C3 plants but, it's not going to cause any yellowing........How about you goggle that up.
800 ppm is not "low". However, if he is under HID lighting. He could run up to 1100 - 1200 and have a real world effective upper limit of 1300 ppm.
Not very many people at all can actually deliver enough light energy to effectively use 1500 ppm!
The other thing is you must run the temps and the RH high to make it really effective to it's potential.....Like if your gassing at 1300 ppm. You need to be running high 80's to 90 deg. F, and have an RH of 78 - 80%