The pot or voltage divider circuits don't work well because as the motor comes under load the current demand will increase and the voltage will drop. If you try it under load you will probably hear the motor speed rise and fall as the peristaltic rollers engage and disengage.
Here are two ideas:
- Run the pump from a 6 V battery.
- Drop the voltage using diodes.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Using diodes to drop voltage.
Diodes have a much more constant voltage drop at varying currents. The circuit of Figure 1 shows a means of getting a range of near-constant voltages very easily. Allow 0.7 to 0.9 V voltage drop per diode. You haven't given us a motor rating so I've assumed it's small and that the voltage drop is 0.7 per diode.
Note that if you test with a multimeter the last two diodes on my schematic they would show 9.2 V as there is no current passing through them and, therefore, no voltage drop.