A peristaltic pump is used to pump fluids. Its main advantage is that the fluid never get in contact with mechanical parts of the machine as it is contained within a flexible tube.
This peristaltic pump was realized for "Hydroponic L.I.S.A." (http://www.thingiverse.com/thing:414271) but it is possible to adapt it for any other machine.
The stepper is a standard Nema stepper for 3D printing
4x 634Z bearings
2x 608Z bearings
1x M8 threaded rod
2x M3 screws to fix the stepper
1x polyurethane tube (the one used for slingshot, outer diameter 6,5mm; inner diameter 2,5mm)