i asume volt-tic is a handy little potential tester you can get at any hardware store. it looks kind of like a highlighter with a plastic tip, the one i have is made by greenly.  you hold it at something, usually a wire or socket or something that your going to work on and if there is enough ac voltage in there it turns a light and beeper on.   
 When I had a failed cord my whole tank lit up no mater where you put it, with that 20-30 v of stray voltage it does not light up or just gives an occasional little bleat. 
 
you said something about grounded potential,  voltage is any potential more/less than ground so if there was enough voltage in the tank and you stick your body part in there and your body part happens to be touching a better ground than a nice dry rug you wont just feel tingling/sharp pain in the water you will get electrocuted until the breaker blows, if it blows. 
		
	
		
		
		
		
		
	
		
		
	
	
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