Is this diet healthy, and does it work?

I was told about a two day diet in which you eat a whole melon over the course of a day, drink lots of water, and the only meat you eat is fish. Supposedly, the people who lost the most weight ate tuna. I can't remember exactly, but it was something like 10 or 20 pounds you were supposed to loose in two days. To loose so much weight so quickly doesn't sound healthy to me, but the meal plan seems healthy enough. Thoughts?