Depending on the fish content of the 180, yes 3 AC101's can be ok.........For example, if the tank was filled with plants, and only plants, the 3 101's would make the bill.............I actually used 3 101's on my 9 foot 180 at one time.............I planned on adding a DIY wet/dry filter.....................I currently use 2 101's and DIY wet/dry on my 125g................Do 50% water changes evry week..............