It doesn't crack off. As the water circulates through the system, inside one of the pipes, is a gizmo called a T-cell. The T-cell continuously attracts the sodium/potassium, and converts it to chlorine, which is released into the same flow of water. It's a continuous cycle, until it's turned off. During the summer, we run it about 10 hours. That's sufficient to supply the amount of chlorine we need, on a daily basis. In the winter, 6 hours is all that's needed, due to the lower temperature. A reservoir would be too much salt at one time, overwhelm the T-cell, causing build up, which would render it useless. Like I said, I usually add 1/2 a bag every 3 weeks. In that much water, you don't even know it's there.What happens to the sodium or potassium when the chlorine is cracked off? And why is the salt added directly to the pool rather than to some sort of "reservoir" for the chlorine generator?