not true, you wont over skim but you wont get a consistant skim, i had a vertex in 180 on my current tank, which the skimmer was just too much for my system, even heavily stocked, went to a little smaller skimmer and pulling as much as the bigger skimmer, but a constitant head of foam, actually have tried it several times with different skimmers, you need to find a skimmer that is size appropiate for your system