HTG supply filters suck unless you are growing tomatoes or something with a low odor. I had a 4 inch and a 6 inch and replaced the 4" with a Phresh Filter and I can run at full blast with skunks and Kushes constantly while flowering and never a problem. I use a Can-Filter for my 6". I also tried Phat filters which work ok but only with the fan at half power, if I used them full blast they would not filter good enough and my whole neighborhood smells like a skunk. heh
So take advice from me who did not take everyone elses advice about spending the money for carbon filters or you will regret it. heh Good luck.
And I recommend
http://www.plantlightinghydroponics.com/index.php for all your filter/fan needs and they also have some of the best prices on lighting. If you do order and want in a plain unmarked box then make sure to request it or it will come in the manufacturers packaging.
1 more thing. The black valueline fans are better made and much quieter than the green active air fans. The green fans are loud as hell. heh