my reef devil.
it collects a half cup every two day's the stuff it pulls out is very thick and dark.
it works best when it's neck is kept clean. if I don't clean it, it continues to work, but the crap is left stuck to the neck in a nice thick layer of dark green sludge, yummy.
the reef devil is a downdraft skimmer rated for a 120 gallon tank, it's a bit underpowered for my tank. there are bio-balls in the black intake tube which chop up the bubbles being sent in along with the water.