Keep in mind... unless you're at around 7,000+ lumens per square foot, and about 85-90 degrees fahrenheit, it's basically biologically impossible for the plant to benefit from increased levels of C02. They simply are not photosynthesizing fast enough if you have less light, and lower temperatures.

From my experience, in most grow cabinets/rooms... there is only enough light to allow the plants to photosynthesize normal levels of C02.