Figuring out some numbers for a mini pump
There are two serious problems with your approach, which you will need to change in order to end up with a sound solution.


If by "9v battery cable" you mean that you intend to use a 9v battery, this is highly unsuitable for motor applications given its low power storage capacity and high internal impedance - without extreme care to use sleep modes, it probably can't even run the Arduino alone for a week.  It would be much more appropriate to use a larger capacity lower voltage source - 4 or 5 C or D cells for example.  A power supply could be another option, though given the water involved, be very careful to pick an isolated one and use a ground fault protected outlet.
Resistors are a poor way of reducing the supply voltage to a motor, as they are not only wasteful, but the applied power will depend too much on the load.  A professional solution would use pulse with modulation to apply the supply voltage at a reduced duty cycle - something you can try with one the various motor shield, but you will need to be careful not to overheat the motor by using too high a duty cycle.  


Probably your best bet would be to buy a pump with a 6 or 7v motor (as is showing up in the associated products at the bottom of that page), or even replace the standard-form-factor motor in that pump with a higher voltage one.  A 7v or so supply voltage can then be a compromise solution for running both the motor and the arduino.