Drill hole at the return pipe where u want to water level at yr main tank to be if everything stop. The reason being when yr return pump stopped for whatever reason, there will be back flow. As the water get sucked back to the sump, yr main tank water level will drop and once the water lvl dropped to the drilled hole lvl, air will gush in to stop the back flow. Hope this help and u may to surf u tube to see the explanation.