I would stick with the pp 8's on a 75 & probably even a 120. I have a pp15 & a pp8 in my 5' 150 & I have to keep the 15 turned down a good bit or the poor fish don't know what to do. Bent must have more control with the jb wave because I couldn't imagine 2 15's on a 4' tank. I like to run them in else mode & in that setting it goes from low to high no matter where the flow is set at on the controller, so if u oversize them by to much u can't run else mode. I had to change out a pp8 to another pp4 in my 40 breeder so I could use else mode on the controller. If they are oversized u have to run them in wave mode, constant flow like a normal power head or in c mode where it rotates them on & off at seperate times. That way u can have the flow turned down on the controller.