Air Products provides this conversion table
- assuming 300psig, 3.5 l CO2 would be approximate 3.5kg CO2
That table is of no use, 300psi is of no interest. CO2 in cylinders is supplied as a liquid and to get liquid CO2 at room temperature pressure will be 55bar about 800psi. Liquid CO2 is sold by weight, they weigh the empty cylinder and add the required weight. The cylinders are only ever filled to a maximum of about 2/3 of their volume, so a 5 litres cylinder is filled to 3.75Kg (3.75l) to allow room for expansion if cylinder gets considerably hotter than room temperature.
You need to weigh your cylinder + regulator before CO2 and after to determine how much CO2 you are using in one lighting period.
I get through roughly 20gr a day in 180litre in 8 hours which is 20/180/8 -> 0.014 grams per litre per hour.
Assuming you mean 3.5Kg CO2 cylinder, for 9 hours in 220 litres and using my CO2 consumption -> 0.014 x 9 x 220 -> 28g per day, so 3.5Kg should last 3500/28 -> 125 days.
You are using 3500/30 -> 115 gr per day.
So you either have a leak somewhere or your CO2 injection method is very questionable, but I would expect to get more than 30 days from 3.5Kg CO2.
Looking what an intense atomiser is, reveals this is the issue. You will not be able to get any decent level of CO2 in a large tank using just one of these atomisers, most CO2 will simply be lost, especially at high injection rates. For large tanks, you really must use in line injection or CO2 reactors so as to not waste most of your CO2.