The GFI would not see current that went out the hot and returned through the neutral. That would look like any other load.
If there's one thing the trade has taught me, though ... try as you might, there ALWAYS seems to be a ground path somewhere. I've lost count of the 'tingles' I've had, when I thought I wasn't touching anything conductive.
At maybe $12, I figure a GFCI receptacle is cheap additional protection for the aquarium. Mine certainly has one ... and my house is two-wire (no ground).
