The speed in which these ballast are made I'm not surprised there is a margin for error, I'm sure there are quite a few that make it through with problems.
I lost all my livestock to a tripped GFCI, I try to limit what gets plugged into a ground fault outlets. Now, I ask myself, "If this piece of equipment shorts out, would I die if my arm is in the water?" That usually limits it to heaters and AC driven submersible pumps. If the answer is NO I don't use a ground fault interrupter.
The UV is questionable because it's low voltage AC and the bulb has a slight potential to come in contact with the water. I hope the new ballast doesn't trip your plugs but if it continues to, I would plug it into an outlet that is not protected.
I never had any luck with the 15A, you're right to use the 20A GFCI's. I tend to purchase the more expensive ones because the cheap ones always seem to be more of a problem than they're worth (I don't know why, but I found that out the hard way). Hopefully your contractor didn't opt for the cheapest home depot outlet.