GFCIs are very sensitive tripping on mere milliamps of current differential between line & neutral. That's one of the tradeoffs in using them & being safe around water with electricity. All it takes is a slight fluctuation, whether it be lamp, socket, ballast or even a power surge from your power company. Seems like you've pretty much narrowed it down to the lamp (bulb) though, since the Phoenix caused you no issues. There's often some black magic going on inside electronic MH ballasts to drive the bulbs and no two bulbs are created equal, a little more going on in an MH bulb than a conventional filament bulb. Troubleshooting such issues can be like chasing a ghost. One could argue that no component of aquarium lighting is actually submerged or in contact with the water, so a GFCI may not be needed. But there are too many variables with each person's individual setup, so GFCI is a great idea by default for lights.
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