The RFP12N10L MOSFET is not suitable, it has a very high on-resistance.
A pump motor drawing 2A at rated speed will require perhaps 20A stall current at turn-on, beyond the
capability of that MOSFET (unless you water-cool it!).
You need a logic level MOSFET rated at <=0.02 ohms on-resistance, on a small heat-sink. Do not try
to get a 100V MOSFET for a 12V circuit, get a 20 or 30V one, on resistances are much smaller for low
voltage devices, and on-resistance is everything.
You must use a free-wheel diode across the motor, this is a hard requirement, otherwise you'll simply
fry the MOSFET and perhaps blow up your Arduino.