Hey guys I'm new here and I am in the process of getting my stuff togheter to start my first grow. I am really into DIY so I try to build as much as I can and buy only the essential things. I found some activated carbon on the web and I was about to order some to build an odor filter but I'd like to know how long before I need to replace the carbon. It would help me select if I need a 5 pound or a 10 pound bag (or more). I am building a 20 plant grow under a 1000 HPS in about a 8x8x7 room. Also how can I calculate the fan size I need ? Thanks alot!