I replaced the original "tensioner" with one from a trail bike...i also replaced the crappy Chinese chain for a KMC 415H.Since then everything has been perfect....no stretch and never and adjustment.
Their two 12V LED "Monza" driving lights with 29 LEDS in each.I got them from K-Mart.
As driving lights u can't take them seriously(cos they spread out too much) but as bicycle lights their fine.
I have them connected via a 3 way switch...off/1 light on/2 lights on.