If you well works like mine did then here we go.....there should be a pump typically attatched to the transom area for drawing water to fill the well. As the well gets full the overflow drains the excess water out. There should be a valve on the fill line, whether attatched at the pump or inline or at the point of entry on the tank all depends on the manufacturer. If the valve is turned one way the pump fills only, if the valve is turned the other way it drains back thru the fill line without the pump running. The valve controls fill only or drain. That is the way mine have worked in the past...see if your is not the same. Good luck