as an electrician i would highly advise against on, gettign a probe you're more then likely going to kill y our system with electricution.
stray voltage sitting in the water is making the fish the same potenctal when you add the probe you are finishing a circuit which will pull amperage through. the amperage going through the system is watch will do all the damage so frankly as far as i'm concerned a grounding probe is only good for the manufacture who is trying to make money off you.
if you're worried about shocks and voltage leaks then invest in a GFI outlet to plug you electrical into, what will sense a difference in current which will trip the GFI and let you know there is a problem somewhere.
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What you say sounds logical. The shock I felt without it is more like a tiny static electricity not like a strong bolt that'll make you jump or anyting. I ran a prob in my tank. Never electricuted any of my fish or coral. Maybe a waste of money but were talking a few bucks not any great amount of money.
the post is correct when sayign the best way is an arc fault circuit breaker as they sense any leak of voltage and trip right away they will also trip when switchign higher amperage switches as the switch will actually arc when opening or closing. if you have an arc fault a gfi is also redundant as the arc fault will sense a short or fault and trip making the gfi pointless as it senses a difference in line current and return current in the neutral