I'd get a phresh filteR. Do not go cheap on eBay or amazon. Get a Phresh brand. They have 4" & that'd be best for you probably. Get as long as you can afford. I'd reccomend buying a 6" inline fan and getting a 6"x24" Skinny style phresh filter.
This is basically the only one I can find though. 4"x12" - $80. Do not grow without it in an apartment.
http://www.amazon.com/gp/aw/d/B007Z...6429&sr=8-4&pi=AC_SX236_SY340_QL65&keywords=4"+carbon+filter&dpPl=1&dpID=51xev0iYUuL&ref=plSrch