Updates 2012 : After practicing Aquaponics for some times, I do not use a pre-filter anymore.
Usefulness of solids waste in aquaponics setup is much debated, there are people that say its required and otherwise, what ever it is, if you need to remove some of the waste here is one method that I employ.
Quote from http://www.aquaponics.com.au
"When solids are allowed to enter grow beds, over time build up occurs and can cause anaerobic areas within the grow beds to develop, which in turn causes toxic substances to develop."
Above diagram shows the construction of the filter. This is a new design that I made, previously I use similar setup but with a liquid swirl at the bottom. This one is still under evaluation.
Water and solids entering the filter will need to make a sharp turn to get out, solids being heavier and sluggish in its flow will tend to flow straight down, hence separating it from the water out.
To remove the solids, just open the ball valve at the bottom, make sure you have a bucket or a hose ready.. what flow out may not to your liking.
Pictures of filter follows;
Above filter, more like a water/solid separator is not as efficient as a filter using mesh or gauze or any filtering medium. I design this one for only one reason, no messy things to bother. It did not have any filter medium so nothing to change, just drain the tube once in a while you are done.