I would question whether this was an "ACTUAL" potassium deficiency or a "perceived" one.
The Symptoms of Potassium deficiency include......
Nerve and muscle dysfunction
Heart arrythmias (rhythm disturbances, palpitations)
Poor kidney function
Low blood pressure
Vomiting (also increases rate of potassium loss)
It would be interesting to know which of these symptoms (if any) you have experienced on your previous fast?
Of course some of these can be "characteristic" of the fasting state, such as muscle weakness and heart palpitations to name but two.
My own view is that if your pre-fast diet is highly nutritious, and has been adhered to for a long enough period of time (say several months) then the chances of an electrolyte deficiency would be minimized and unlikely to occur, unless you have a malabsorption problem.
Do let me know.