I have always wondered if it is possible to avoid using any sort of tensioning whatsoever for roller chain. If you properly calculated the measurements for the location of the sprockets you would never need to tension the chain. I reference this website:
http://www.battlekits.com/
This drivetrain uses no tensioners, though granted, they do use #35 rather than #25. Smaller chain stretches more than larger chain. I really like the Battlekits design and function, and simplicity. What do others think about not using tensioners?