Eheim pumps are 240 V. Try searching eheim relay. As for changing the voltage for the pump, try to see if you can get hold of a lamp dimmer spec at 300 W. I don't know how well it would work though, as a lamp is considered as an inductive {charge? my english is failing me here} and the pump is capacitive.

But if I were you I wouldn't mess with it. I had once a 2300W pot for a heating element so that I could control the power transferred to a liquid, over time the transmission lines became dust covered and started a fire. Thank God that I was home and noticed that the power went out.