in a 3 foot tank, it should push it to the end and make for some back flow... in a 4 foot tank not so much... but they are not made to work like that anyhow... if you have one, it should be centered on the back wall, and if 2, one at each end, and ideally, you should have 3, but thats not practical for most... they work with each other, and the contact of opposite flow from each end, is waht makes for the chaotic and random flow that everyone likes for their sps...