I ground all my tanks period.
It is just to easy for stray voltage to find it's way into the aquarium thou my tanks are saltwater it happens in fresh water tanks also.
as far as testing for stray voltage I look at it as a black and white issue or on/off, I know this sound crazy but for my I can feel for it!
DO NOT ATTEMPT AT HOME-ok I said it.
I find that if I ground myself (like I do every morning) bare foot feed the fish in the AM--One day I noticed a slight poke on Quitical?--then confirmed my findings with a voltmeter. I then unplugged light, pump and power head one right after another till I felt no poke, Turned out to be a faulty power head (saltwater tanks are much harder on equipment. This is a very low tech way to find the problem but it works.
Also another great reason to ground all tanks. Now honestly how many of us are guilty of simply setting our lights directly on top of our tanks....I am guilty of it.
"Hole in head" is thought to be caused by stray voltage, and or poor diet.
Bottom line GROUND your tanks.