The initial tank dosage of Forget About It For Boats takes into account the accumulation of years of sludge and struvite in the bottom and sides that have never been removed from the tank. By double dosing the tank the first time it ensures that the quorum is captured by the beneficial probiotic bacteria in Forget About It For Boats. See the Faq Forget About It For Boats outcompetes other bacteria for more information on how and why that happens. After double dosing the first time, the 1 to 2 inch layer of sludge in the bottom becomes saturated with beneficial probiotic bacteria feasting on the rich food supply. The probiotic mother culture will continue to live and feed on the sludge until it is completely consumed. It will continue to outcompete the harmful odor causing bacteria as long is food is present and actually grow to consume any new organics flushed into the tank. The dosage is cut in half because the tank already has a probiotic mother culture living in the tank and the tank is now cleaner so not as much is required to get the same results. Yes, it really is that good.