Most canister filters, except the old style Eheim, have both hoses going to the top of the filter. Each one has a different need for how high the water level needs to be above the top of the filter. Depending on the model it can be anywhere from a few inches below the water level to a couple of feet. The height will determine whether or not the impeller on the filter can run quietly. If there is not enough height, the impeller will make a crackling / grinding kind of noise that means that the impeller is cavitating. If you run a filter and allow it to continue to cavitate, it will eventually destroy the impeller. For that reason alone, I would observe the minimums that the manufacturer recommends.