Excel is supposed to be dosed at 5 ml per 10 gallons, after a water change. Double that, as some do, and that is 10 ml per 10 gallons, or 1 ml per gallon. A gallon is about 4000 ml, and one ml is about 20 drops. So, 1 ml per gallon is 20 drops per 4000 ml, or 1 drop of Excel per 200 ml of water. That is the dosage you would want to do that test tube test with. Using one drop per 5 ml is a meaningless test.