What do you mean by your don't won't "clear" up? Are there actual particles floating around your tank? Large particles or small dust like particles? Did you distrub your substrate recently? How large is your tank? Is that AC300 the only filter on your tank?
I personally find that aquaclear filters are one of the best all around HOB filters. Stick two sponges in one, and you got an excellent mechcanical and biological filter (given time of course). In any case no matter kind of filter, you need to give the filter some time to filter out floating particles in your water. On my 66 gallon cichlid tank, I have a fluval 404 (to be replace with an Eheim that I'm eyeing), an AC200 and an AC300. Both AC filters have two sponges in them. One time, I rearrange the entire tank, and really stirred up some dust from the substrate. It took a couple of hours for everything to clear up... and this is on a tank that has a good amount of filtration on it.
HTH
-Richer