I have a Knop HOB on my 55g reef. I am okay with it, but not thrilled by any means. It absoultely will overflow, produce tons of wet foam for no apparent reason, and is basically inconsistent as far as I can tell. When you get it just right, it skims very well, pulling out dark to medium gunk at about 2/3 a cup per day, day and a half. One more note, it is a real pain to get the cup off to dump and clean, so I have to use a turkey baster and empty it that way, then take a rag and clean out the inside of the tube. Good Luck,
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God Bless,
Jerry