I've done a search and read many threads on Co2. Unfortunately, money is tight right now. Could someone counsel me on a cheap way of adding co2 until I have enough cash? Here is some facts about my setup:

1. 10 plant drip system (Rockwool, nutrients etc.)
2. 400 HPS and a 65/500 Flourex flood light (first light i bought.went cheap on light I know, then realized I needed HPS
3. Grow space is a closet with a door on each end (Length 9.5 feet, Height 8ft):smokin:
4. Other equipment I have is a humidifier and an air filter/ionizer

*Note-Still waiting on shipping for lights and seeds

I know there are other things I need but will buy them soon. Any advice is appreciated