I've had a 6', 140G. tank sitting in my cantilever for a year now without a problem. Consider that it will weigh in at just over 1000lb's - or the weight of five big guys. Will the cantilever collapse if five big guys stand in it? As suggested the tank sits on a load bearing wall and if your tank is say, 20" wide only 16" actually protrude beyond the wall itself. That 16" is
cantilevered which means it is supported by the load bearing wall.
here's a pic:
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Kawasaki KR350