I also was worried about electrolyte imbalance after a coffee enema.
As a preparation, I simple just did a water enema mixed with a spoon full of salt to clean the colon and avoid dehydration. And after the coffee enema, I actually drank 4 cups of water w/ Sea Salt to make sure I`m not dehydrated.
This is very simple advice, but it seemed to have worked for me. I feel fine and the constipation has gone away.