Why is my aquarium water cloudy?

It all started a few days ago, when i did a water change. I siphoned out about 2/3 of the old water, which was perfectly clear, but I figured it was about time to do a water change since I hadn't done one in almost three weeks, then put a new filter pad in (I have a penguin 200 in a 20 gal freshwater tropical with 6 female bettas, live plants, et cetera.) It was cloudy as soon as I put the new water in, but I figured since it did that last time but cleared up it would do it again once everything settled. well, it's still cloudy, and I was wondering if I should get a chemical at the petstore to clear it up, or put something else in there or do what? thanks, if you answered!