How many gallons is that tank? Are you testing your water? If not, that's going to offer the best insight - API makes a freshwater test kit under $30
http://amzn.com/B000255NCI or several pet stores will test for free.
If you have high nitrates you may just need more frequent water changes, if you have high ammonia, you need more bio-media.
The cloudiness makes me wonder if you're cycling... do you clean your bio-media in tap water? How long has this filter been setup?
It seems like the FX6 should keep up with your tank adequately, unless the volume is more than I'm guessing.
Small suggestions: Move your input and output to opposite ends of the tank to improve circulation.
Consider your lighting, both artificial and natural: The greenish tint may suggest algae growth?