Hey whats up guys Im getting a 400 watt cooltube in about a month and a half
and Ive been doing research and I was just wondering how many CFM I would need to cool it. Im also getting a 36"x20"x62" grow tent as well. This will be my first grow with HID lights so any help will be greatly appreciated.

are you going to do a closed loop pulling air from out side thru the tent then back out. are you pulling air from inside the tent then out
are you pulling air thru a carbon filter then the cool tube then out? If you run it as a closed loop 300cfm should do ok, if you are pulling thru the tent you might have some issues with plants drying at high speeds. I would get a decent vortex centrifugal inline fan and a speed controler. I say buy oversized and control it that way you can go bigger.

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It shouldn't be that hard to grow they do call it weed for a reason.