outsider:
How much current does your pump require? What other things is the Arduino powering? Can you post your code?
On full power, with 12V it would draw 2A, but I'm powering it with 9V, so something like 1.2 - 1.5A max.?
The arduino is also powering a LCD Display which I didn't include in the sketch.
The code is pretty simple right now, just reading the value of the potentiometer and writing the value to the pin11 with analogWrite(pin11, potentiometerValue/4)
Plus the code for the Display.
I'm not at home right now, I will post the code in the evening.