Updates 2012 : After practicing Aquaponics for some times, I do not use a pre-filter anymore.
This is another variation of solids prefilter that I post earlier. The different is I stick a swirl adapter to swirl the water causing solids and liquid separation.
Diagram above shows swirl adapter, the rest is self explainatory.
Photo of swirl adapter, if your filter casing is big, you could use two elbow at the end of the adapter to cause the swirl.
If a "U Turn" won't do it,, just turn the water around to get it loose.