I have no problem with 'hard wiring,' assuming that there is a disconnecting means within sight, AND it it STILL on a GFI.
I've seen a number of home fountains, and the only one that is even close to being code compliant is the one at my house - and I have NO nuisance GFI tripping.
The most common error in fountain installs involve the power cord. The corn is buried in cement, then travels underground ... only to pop up at the last instant near the receptacle. While this might hide that ugly cord, it is a BAD idea.
The cord material is not very resistant to moist dirt or cement. The rubber jacker deteriorates very quickly ... even if the same material lasts for years on your outdoor extension cords.
Likewise, "gardeners" and "fountain guys" seem to have no qualms about splicing sundry cords together - often creating a wiring error - and using duct tape to hold the twisted wires together.
The GFI just might be doing its' job.
Here are a few tests you might perform, to diagnose the problem:
My favorite is to use a megger on the wires; this will find any bad insulation.
I have a test cord whose outer jacket has been removed, to allow me access to the individual conductors. On a non-GFI circuit, I can also check the circuit with my ammeter. Not every ammeter is sensitive, or accurate, enough to measure such a small imbalance. Simply put, you ought to have the same amps going 'in' on the black wire as come 'out' on the white.
And, let's not forget the obvious: the receptacle itself. Plug something else in, and see if it trips. Your problem may be something as simple as an unused screw not tightened down.
