Have you got a speed controller on your fan?
Maybe at 100% its smelly and dialed down it isn't.
Just a thought. If not I'm all out of ideas.
J
I was thinking that too but a six inch fan on a 10x20 filter should be ok. Hmmmm, i'd try and slow the fan down some anyway. And straightening your duct like burgertime said will only increase flow so slowing the fan some may be necessary if you do that anyway.
I dunno but if you are in a very high traffic area like aptmts. which sounds like you are you may want to look at an inline ozone generator as a failsafe in your exhaust. If the carbon doesn't catch all of it the ozone should. The size of your tent.....it's gonna stink bro.
also a recirculating carbon scrubber in your tent may not be a bad idea.
how many inches of carbon are in that filter? I have found cheaper filters to be just that......cheaper.
edit: you do have the joint between the fan/filter caulked yes?