I realize you're a more experienced at growing mj, and I definitely defer to that, however I spent 5 years filling tanks in a local pb field, and had to be certified in carbon dioxide, compressed air up to 5000psi, and Nitrogen gas, and a 50 lb bottle of co2 in an enclosed grow space would put an elephant down. Co2 is lethal at a dose of 5000 parts per million and can cause unconciousness at levels as low as 3500 ppm. That means that for every 'liter' of air, it would contain 5000ml of co2. That roughly translates into 2% of your breathable air and if you're releasing that in a grow space, say the size of a closet, you're going to at least knock yourself out, if not cause cardiac arrest. Now, consider that co2 is a heavier than air type of gas, shown that it's sold by weight, and not volume. This means that standard ventilation, which tends to draw ambient gasses off, won't remove the co2 from your grow space. Co2 settles, it doesn't blow away and dilute.
Here's the MSDS if you're interested;
http://www.uigi.com/MSDS_liquid_CO2.html