I assume you have the red intake and output valves turned all the way open? I gave up on canisters and converted every tank to sumps except my fishroom which is all air. I absolutely loathe cleaning my FX5/FX6 canisters. I was replacing the Fx series motors every 3-4 years. The FX6 has a lower operating voltage which saves you a few cents per months and as mentioned they’re interchangeable between the two models.