1.8 Ohms sounds reasonable, in which case the start/stall current, operating from a 12V supply, would be around 6.7 Amps. A 10 Amp motor driver should work.Or, follow this tutorial for a MOSFET driver http://bildr.org/2012/03/rfp30n06le-arduino/Contrary to the advice given in that tutorial, you should use a logic level MOSFET, like the one recommended.
Optical sensor to the Single-head pump
So I would need to run the sensor in-line with the mosfet between the gate and the arduino?
And if I understand correctly I attach that separately to the Arduino and use the Arduino to read the optical sensor and then based on the reading of the sensor the Arduino talks to the motor?
yes I zeroed in both ends of the scale. it seems like it is 1.8 ohms.
I think that the sensor is wired like this:Google "Optical Infrared Water Liquid Level Sensor " and this is a common 4 wire diagram.