A circuit could only be made is there was actually stray voltage in your tank. And I see, I never addressed that issue.. Stray voltage is , I suppose in simple terms a leak in the voltage being requested by your equipment in your tank (only equipment in the tank can leak voltage (in this case the only things capable of this are the 3 powerheads)). There are a few reasons I can see this would happen, 1st poorly made equipment, 2nd damaged equipment. A grounding probe I suppose would be used to give the voltage an "out"..
The statement I made before was. I have no idea what that voltage could do to fish. And I also dont know what it does once you ground it.. I was just trying to give you some more input from a (I suppose) land based observation.. I am sorry I can't be of any more help..