venting from the loft/attic

Im setting up a small growbox in my loft with about 6 plants just over 4 weeks old. they are sat by a window at the moment and doing alright considering the shit weather in the uk! anyway i have a flexible ducting in the loft going from my bathroom extractor fan to a hole in the roof so a can use that but how do i actually "suck" the smells out? the ducting is about 6 inches internal diameter. any ideas?

you have to go to home depot or something similar and get an inline cfm fan. a 6" should work nicely... there are actually quite a few threads on your question... just do a quick search