I've done it myself many times. You can lose up to 10 pounds in one day if you're really overweight. And yes you can lose up to 20 pounds in a week. 6 pounds is water and fecal matter that doesn't count as anything. If you eat lettuce that weight will come back. You gotta have that back. Now you're down to 14.
Your metabolism will be so severly slowed that you'll only be able to keep that weight off by eating so little, I'd say only fruits and vegetables for a month, to get your metabolism back up to speed, that life will be rabbit like.
If you want to lose 20 permanent pounds water fasting, you'll need a day of fasting for every pound. Say 3 weeks for a permanent 20 pounds. Then you'll have to eat like a bird for the following 6 weeks until your metabolism catches up. Then you'll need to limit yourself to 1200-1600 calories a day of wholesome food to keep it off, depending on your activity level, age (hormones, can't live without 'em, can't live with 'em), gender, darn hormones again.
Quite a task. The whole disciplinary process will be a 3 month process. Owwwwwwwwwwwwwwuch.