Re: 21 day fast maybe 30 days

I would only demur over your choice of foods for your last pre-fast meal. It is far too carb-rich for my own tastes & comfort, and also potentially inimical to safe fasting, coz it immediately puts your organ & muscle tissue in the firing line. 

Before any fast (especially a prolonged one), you want to put your body in fat-burning mode, which basically means eating as much healthy fat as possible. My two preferred strategies are:

A. Gk salad-based
1. Your favourite main salad ingredients (as in lettuce, cucumber, tomatos, celery, peppers, onion, etc)
2. Feta cheese
3. Avocado
4. Any crushed fatty nuts, such as macadamia, pecan, cashew, walnuts, etc
5. Olives - green or black, it doesn't really matter
6. Drizzle with a healthy oil (avocado, olive, sesame seed) and any other dressing you like, such as a vinaigrette, cider vinegar, etc
7. If you like it, add a dollop of full-fat salad cream, or, better still...
8. Thin some peanut butter with the oil and/or vinaigrette & pour over salad as a dressing.

And bingo, you're now set up with one of the healthiest fat-rich meals on the planet - full of phytonutrients, micronutirents, antioxidants, roughage - you name it.

B. Fry-up
1. Fried egg 
2. Fried haloumi cheese
3. Fried multi-grain bread
4. Fried onion (or m/waved if you prefer), or garlic
5. All frying done in a healthy oil, such as grape (not rape)seed or rice bran.
6. You can add your favourite sauce if you wish, eg ketchup, bbq, brown sauce, etc, but be aware of the high sugar content of these sauces. A healthier (lower sugar) alternative is tomato paste (aka salsa).

NB - The onion and/or garlic are not optional extras. They are essential for preventing any saturated fats from attaching themselves to the delicate epithelial lining of your blood vessels.

Beyond that, you need not be too concerned about potassium depletion, even for long fasts. If you plan any strenuous exercises, however, you would be well advised to supplement with any recommended electrolyte-rich drinks. Just make sure they are not sweetened, not even with artificial sweeteners.

Hope that helps.