Maybe I don't understand the set up correctly.
Someone draw me a picture.
Is the output of the returns 6" below the water surface?
If it is then putting a T with a cap and a hole in it will not stop the tank from back syphoning.
It will work more like a durso and gravity will push the water level in the tank down until it is below the return and that will stop the flow of water out of the tank.
Unless I am not understanding the setup correctly.
If that is the case then ignore everything I said and I will go back to sleep.
