ATO, Stop siphoning?

[banditpowdercoat]

I am building a ATO for my tank. Have a 24V control board built witrh relays to operate the pump. But, when the ATO level is higher than the sump, how do you stop siphoning? I guess a Peristalic pump is the only option? Have been told that people use a powerhead, like maxijet mp1200. But that would allow siphoning, won't it?