However Drea, the test that was performed on several skimmers including the precision marine ,bubble king,etss ,ect. which were in fact small internal skimmers on a "scientific- none living reef system". Therefore the percentage of TOC's and DOC's in a "real" reef set up especially a large reef packed with corals and fish and using "external" large skimmers would have been certainly higher. Also , as I have stated before , using an external skimmer , especially an "recirculating - counter current skimmer- MTC Mazzei venturi design's" will give you much greater flexibilty in controlling the amount of "feed water" into the skimmer as well, total control over how much "air" you want to use to mix with the "feed water" into your skimmers column. Internal skimmers don't give you anywhere near that kind of control and water levels in you sump are very important to maintain the internal skimmers performance on a day to day basis. I am also biased to using only tall and decent diameter skimmer columns over short "stubie" columns (internal design's) as you do not get the dwell/contact time which is very important for removing larger molecular proteins that need good time to attach themselves onto the "right" bubble's.