I have posted on this topic before, and I am one of the people who says that:
A ground probe WITHOUT a GFCI is bad. You could have current running through the tank, and you would never know it ( because you wont get shocked as much anymore ).
If you have a GFCI AND a ground probe, the GFCI will trip immediately if there is any current through the probe/tank & you will know something is wrong.
ALSO - you should definitely have a GFCI on your tank ( lights can be an exception ) for personal safety reasons.
Newer houses have GFCIs built into the circuit breaker panel.
You can install your own GFCI in the socket that you use for the tank, build one into a power panel - like this:
( GFCI is on the far right & protects all the other sockets as well )
, or buy a power strip with one built in ( this is DIFFERENT from the circuit breaker on the power strips most of us are familiar with ).
However the best combo is GFCI & ground probe.
My GFCI has already saved me from a fire. I accidentally soaked a 'apparently-sealed' pump plug in SW over night. I dried it off thoroughly, and when I plugged it in it tripped my GFCI.
If the GFCI had not tripped, the current thru the salt was not enough to trip my circuit breaker, and eventually the plug would have caught fire.
Stu