Tripping a GFI does not necessarily mean a voltage leak, you don't mention being shocked. Their are some probably causes, send it to me and it will be taken care of, they have a two year warranty. However, I can honestly say I have never seen a Tunze release stray voltage into an aquarium, nor have I ever heard of such a thing, our pumps are grounded and very well made. I will test it on a GFI as well as a watt meter and find out what is going on, all a GFI trip means is that power exiting does not equal power entering the unit and this can be as simple as a weak spot in the coil delaying the transfer of power.
Send it with a letter detailing how it was being used (timer, constant on, etc) and when the problem started. I will take a look at it, run a few tests and provided it was not run dry or seized for an extended time or used incorrectly, it will be replaced or repaired under warranty.
Tunze USA
1304 Monica St
Austin TX 78758