What kind of switch would I need to vary spped on my pump?

Dimmer switch @ Radio Shack should work. I believe they vary the voltage which would change the pump speed. -Chuck.
A dimmer is for an AC motor. The pump is a DC motor. A pot is the right thought, however you'll need one with a 250 watt+ rating. Not practical. The simplest route: Go to a tool repair shop and get a speed control switch, like for a variable speed drill or v.s. weedeater or v.s sawzall or whatever that is specifically for a DC motor. The amp rating will have to be at least what it is for the pump motor. Failing that, you'll need someone (like myself) to design a speed control circuit for you.