Go down to the Ocean...stick one end of your volt meter in and the other to a "known good ground".. .
Thats how much voltage is "OK" in your tank...

(Hint: you'll get a 0 reading)
When I felt my first "shock" I had 14v in my tank ...my buddy had 70v...(before fixing) ..but you would only feel it if you "became" the ground...(wet floor, stocking/bare feet, touching the light fixtures) with your hand in the water...
This is where the GFCI is your "FIRST" line of defence...At the "Instant" a voltage surge is felt it should trip and keep you from getting a potentially dangerous shock...shutting the entire circut down...Make sure it's installed and grounded properly
Then the "voltmeter test"...eliminating/repairing the offending pieces...
Then you may still find "tiny" ammounts of "stray" voltage (electromagnetic inductance as opposed to a direct "short" or "fault") those readings are generally in the <1v range... These are the ones I believe are the cause of (or contribute to) HLLE and stress related illness in fish...a grounding probe will draw them off...
Without getting too far off track the "stray voltage" the fish are subject to is "similar" to the problems people have living too close to High Voltage power Lines...
Again the GFCI is the first line of defence and any surge felt (short or fault) whether grounding through you or the grounding probe will shut the system down...
Use both...or at least the GFCI...