I have this same filter It's been slowing down a bunch. I opened it up to look inside and it's pretty clean. What could be the problem? All kinds of junk came out the spray bar when I was trying to clean it so I had to do two water changes. I even took off the tubes and ran water through them to see if something was stuck but it was fine.
Take a look at our CF cleaning instructions--good stuff. The intake and output valves are most likely gunked up like the spray bar and will need a good cleaning. We also suggest checking the media baskets and make sure that they are up to par. A lack of good flow will make a motor work harder and burn out faster.