All you need to set the voltage to the pump to slow it down is a selectable output voltage transformer available from electronic supply places like active tech or radio shack. The controller is the true problem as it will be designed for a certain voltage and likely wont operate properly on less voltage but it should be fine to run it and the pump on 18v possibly even less. I run model trains that use /24v motors as low as 6v for short runs and about 15v for hours at a time. Your amp draw will climb with the lower voltage.
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