Just to clarify, the canister itself has no flow adjustment. But the flow is very adjustable by the valves built into the quick disconnects for the input and output hoses supplied with every new Ehiem.
Adjust flow using the output side hose only, it should be easier on the pump.
Eheim Pimp #459 - 2213, 2234
SunSun Pimp #47 - (Perfect) HW302