tsicar

thanx for the pix and the diagram, smithson. looks like a similar setup to the external one i was talking about, but with added advantage of bypass overflow in event of blockages.
important thing is that it takes the fouled water from the bottom, where most of it settles.
from here to the veg, then on to biofilter, or if you don't want to do veg, then normal septic-tank type system for processing of solids before return to biofilter. (i prefer the veggie step, tho!)

just a comment on the whirlpool: if the tank is circular and you have a circular motion of the water (whirlpool), the worst of the solids seem to congregate at the centre of the tank, so perhaps this should be where the syphon is positioned?
i seperate catfish fry that i accidentally syphon off while removing fungus and detrius from the hatchery tank this way: dump all the leftovers in a bucket and stirr it up, with a shaded area at one side of the bucket and a strong lamp above. strong fry that were accidentally sucked up will congregate under the sheltered part and the weak ones plus all the crap congregates at the centre, and this is easily syphoned off and discarded.