I use these for my returns on a herbie style overflow, they have a small hole drilled into them right at the water line which acts as a siphon break once the pumps shut down. https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Lots of good info in here as well. http://www.beananimal.com/projects/s...ow-system.aspx