Is it possible u are drawing more current then what the GFCI can handle? Used to happen at my old house all the time. My air compressor in my garage made the GFCI trip so much it eventually fried it.

I built my own arduino controller, allows LOTS of expandability for very cheap. I have an ethernet shield on it to post the stats onto pachube.com that I can access easily on my webpage. I'm using no-ip.com for a redirect because of my dynamic IP address for the webcam. I should probably update my build page to show all the info, there's been far more time sunk into this than I would like to admit. Probably enough to make it worth buying a controller than does this all for me :P.

The outlets are GFCI, not the breaker.

The strange thing is the bulbs will light fine and work for a few days then trip the GFCI. And it doesn't trip at startup, it's always later on.

Like yesterday, it took a couple hours before it tripped.

My fixture is also grounded with a ground connection to the outlet. I may try swapping the bubs around today, but the thing is the Ballast (only 1 year old) is a Dual, and you cannot control each lamp individually. It's both or nothing.

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.

I swapped my bulbs around (still running both Artemis) and double checked that they were seated correctly, and it ran for about 5 hours last night without issue. So let's see how they run today. Hopefully no more GFCI problems.