Hi Don,
I am still learning, but I have had stray voltage in my tank over the years and sometimes it was caused by a bad heater and other times by salt build up or excess water (typically after a water change or major maintenance). In my experience it has had zero negative effects on my slice of the ocean, albeit annoying when I stick my hand in the water and get a little zap. To my knowledge there is not anything that can continuously monitor the stray voltage, aside from manually checking it with a volt meter.
I have always bought the inexpensive heaters (generic brands) off of Amazon and replace them if they go bad for $10-$15.
Hope this helps!
Maz