See SystemCooling for the details of four square solder pads on one edge of the pump's circuit board. One is for pump voltage speed control. Or, you can run your own voltage control via the pumps regular two power wires.
Bottom post and into the next page ...
http://www.systemcooling.com/forums/...pic=3099&st=30
Even though the pump is rated to run between 6~14V, I would only run it as low as 7.5V in my system. Many variables like coolant mixture and flow resistance will determine your actual low speed of operation.
Hope this helps!