Its absolute that a drilled set up always works. No water can overflow either tank, as long as return outlets are at high enough level in maintank.. I am extremely comfortable with the Eshopps overflows because of many years using them. No pumps or problems. The u-tube is designed to always keep siphon by both ends always staying just under water level. It has chambers in inner box and outer box that make sure of it. Again though, return outlets must be near enough surface when things shut down.