A 100 liter tank takes only slightly more than the time between ad breaks on fair go to siphon all the water out...
Was changing water in one of the fry tanks, usually seems to take AGES, this time I threw the hose in and left it going until the next set of ads. And when I went and looked there was maybe 2 inches of water left. hehe, Oops. Lucky fish, 90% water change!