Another suggestion was made to me to put the pump on a PWM connection and allow the Arduino to control the maximum load to the pump. Is this feasible?
dave-in-nj beat me to it 
You can make a switching regulator controlled by the Arduino. Not only will that be a simpler circuit which is easier to commission, if you want to learn about electronics that'll be a great sub-project. As Dave rightly says, if you run a switcher at 30% duty cycle, you'll get 30% of the input voltage at the output. But by using the inductor and diode, you get vastly better efficiency than using resistors, or by switching a mosfet alone without the inductor. Look up buck converter on Wikipedia, and if you want help choosing components, say so here so others can benefit/contribute/correct, or PM me if you prefer.
As you said, get a bigger motor
Ah, that's not quite what I said. A higher voltage motor isn't necessarily a bigger motor, it's just that its windings are optimised for a different voltage. More turns with thinner wire, higher voltage with less current, same power, same speed, same size.