You must stay hydrated, so drink as much water as you can possibly drink. That's a given for any fast. You're going to be losing a great amount of water when you goto the gym and use the sauna. If you don't replace it, that's a good way of killing yourself QUICKLY. So make sure you're getting your daily water intake and UP the normal intake to match the water the sauna will be robbing from you. You also mentioned electrolytes... I'm not sure how long you're going on your fast but extended fasts you will absolutely need to be replacing those. The sauna again will be robbing you of those. Each day you visit the sauna, use pedialite, other than that you should take it every two days.