you could actually get away with using just 1 fan, im using an 4" inline fan with carbon filter combo with my 600w hps and if you hook the fan up so that is exhausting out the top corner, then it will pull in fresh air from the other intake vent(you have to keep the tent air tight), hence not having to use 2 fans. But if you have the money you COULD use 2 fans. Personally i think its a waste of money.