As Boomer says "Your tank - Your choice". The purpose is for circulation so both in rear center facing out / rear corners facing in / rear corners facing diagonally in / rear corners facing front... all up to you and what effect you want them to make - near same circulation everywhere / high/low areas. There is no real propper placement for them just what is pleasing to you. As I said the purpose of them is to circulate the water and wherever you decide on placing them in the tank - they'll do that.