Thanks for the clarification on the potentiometer.

You can control current with a resistor(or Potentiometer) by wiring the resistor in parallel as opposed to the more common series in a circuit.
In addition to this, and since the principal of a CS generator is so very basic, I'd encourage you to assemble a reliable generator first, to work toward more permanent solutions afterwards
This way, you can gain a better understanding of the principal of a DC electrolysis generator, to apply toward your design improvements. 
 

DIY - Colloidal Silver Generator Design - 2a

In the above mentioned design reference, a good first step toward improvement could be the addition of a permanent power source.
Though keep in mind, that each improvement will bring along with with it, added complexity - specifically on the troubleshooting side of things.

As for the diode, the most current control for simple drinking glass generators, range between the .75 and 1 mA - that is a J504 and J505 part respectively
J504 0.75mA current regulator diode
J505 1mA current regulator diode

NB. The Diodes can be stacked together(in parallel) to raise the current threshold should your increase your anode size down the line

- Hope this helps

PS. if you're working toward permanence, I'd recommend a good variable DC power supply(preferably digital), as this will save you save an immense amount of frustration and money - burning-up components