Use a Titanium ground probe and check to see that the circuit it's plugged into actually has a good ground. People have lost tanks due to false tripping by GFCI's.
If you have your grounding wire running through your GFCI (and you absolutely should), the GFCI will trip if voltage leaks onto the ground wire.
You can spring for higher-end GFCI outlets that are less prone to tripping, and that's totally money well spent.