A ground fault is detected when fewer electrons return than go out (adjust this heuristic for AC). So it trips if some current goes through your arm, heart, leg, etc. This does not require grounding to function. However, you should also ground your tank, so if you can put a titanium ground probe (or bike spoke) into your tank, attach a wire leading to a METAL pipe or electrical conduit. That way the GFCI will trip before you complete the circuit with your arm, heart, and leg, which is probably preferable.
