thanks guys, the advice and luck worked!
Ok this is what i did: take a toothbrush (clean and disinfected of course) and take the emp400 completely apart. I brushed the impeller (which had some plant roots or something wrapped around it) and stuck the toothbrush inside the hole where the impeller goes, and cleaned the stuff out.
After throughlly cleaning it, and using hot water and high pressure, i put it all back together and it flows like new. I am sure if i changed the filters it would flow even easier, but i am all out right now.
So thats basically what i did. What causes all that buildup inside the filter? because inside my aquarium the glass and rocks and water are sparkilng clean but inside the filters it looked like gunk was caked in there.
By the way: I took the emp400 out and cleaned it a couple weeks ago BUT i didnt do the toothbrush trick and it helped but not as great as it did this time. So i guess thats the "secret".