GFCI will work without a ground or probe. They didn't when they first came out long, long ago, but they had to change it. Look at your hair dyer , TV, toaster, radio and some power heads and heaters, most don't have a ground plug on it. so they change them. That's was what I was told by a old timer
Take a voltage meter and put one end in the tank and the other on a ground, I think that should tell.
If the voltage is small,(voltage can be from 1 mil volts to millions) A grounding probe will work. It's like grandma old refigurator in the garage, when you touch it. It shock you a little. It's not grounded. Power will find the easiest way back home and that will be the ground wire, not you. But that don't make it right, it's still leaking voltage. Corals might not feel it because they are not grounded but saltwater is a conductor and if it somehow gets grounded it may and if it don't, It's like I said, Would you live under power lines. In fact I think they don't build under power lines no more by law
If you feel it, Fix it.
