I don't think you should fast until you adressed the reason WHY you gained 20 pounds back. 20 pounds is a hell of a lot to gain. It can't all be water. It obviously means that you didn't learn from anything. Now technically your fatter then you were. You lost muscle and replaced it with fat.
If you do this fast lose those 20 pounds and then gain it back. You'll be even fatter.
Stop the cycle before it it continues. Otherwise you'll end up living a life of yo-yo dieting.