it's not the cheapest way but it works great me and my buddy buy a brick of dryice and split it and put it in hot water.the dry ice is solid co2 when put in hot water it turns to gas and fogs your room out with co2 its like 80 cents a pound and 2 or 3 lbs is great for 3 or four plants. when you do it turn off your fans and place the water and dryice above the plants if you can the smoke tends to fall down i love this method