If you want to get technical, read through this:
Reef Central Online Community - View Single Post - Grounding probe? waste of money?
Larry posted this a while ago and is an excellent source of information on whether to use GFCI, AFCI or grounding probes.
Here's the summary if you don't read through it all - but I suggest you do:
"So in summary overall the safest for the aquarist will be the installation of AFCI, GFCI and Ground Probe. The safest for the inhabitants will be AFCI and GFCI without ground probe. This is the reason why many aquarist do not use ground probes (but will always use GFCIs) specially traveling ones that stay away for extended periods of time. If you have a remote alarm system installed that may page you or send you an E-Mail in case of a fault (Like the aqua controller from Neptune Systems) you shall have no doubts and have your Probe installed with your GFCI's)"
Credit: Reef Central Online Community - View Single Post - Grounding probe? waste of money?