Stoping the Syphon

[BlueTang<3]

So i just plumed a tank for a friend and between the pump and tee to the returns there is under 3 inches, once i had the 3 union ball valve with he low stand. What can i use to break the syphon? I typically would not be worried but he likes to move the returns to direct flow in the tank and unless they are completely uncovered they will not stop syphoning. Being i think he is going to lower them at some point and flood his sump i am concerned. Any low profile out there. I also have limited room or i would just throw a check valve in both return lines but the one does not have room.