I have been told that the GFCI is activated from a drop of 5 mili amps in the return current in other words part of the current is going to a ground fault not necessarily to the grounding.
That is 100% correct.
However what other path for current is available here?
Water in a glass tank sitting on a stand that may or not be conductive sitting on a floor that is likely to be non-conductive.
Without an EGC or other fault path the only path for current is line to line....GFCIs ignore line to line current.
Also Iv'e heard talk of all residences requiring AFCI protection being pushed by NFPA
It's more than talk, the 2008 NEC will require AFCI protection for most dwelling unit circuits,