How to hide litter box odor?

I just recently got two kittens, they are great but there poop really stinks which of corse poop is not supposed to smell like roses, so my problem is my house is smelling yucky, I have a covered litter box, not one with a flap because they don't like it, and I am using fresh step litter. I scoop there box more than once a day and change the litter completely once a week. is there anything I can do to stop the smell!! is there any type of litter I can buy that works wonders with odor control?