I'm no expert, but this is what my understanding is based on asking around:
1) Electronic ballasts have to have RFI filtering in place to comply with device regulations. This RFI filter bleeds up to 1/2 amp.
2) GFI doesn't care about surges, etc -- only about a current flow mismatch between hot and neutral.
3) Because there is an inherant mismatch of current with the RFI circuit bleeding energy, the GFI can trip.
This apparently doesn't happen with magnetic ballasts because they don't have the RFI circuit. It can also apparently be hit or miss with electronic ballasts depending on how much current is being bled by the RFI circuitry.
FWIW, I too had my lights on an aquacontroller with a lot of other things. When my new lights tripped my GFI it shut the whole tank down. I am now handling this by adding a DC4-HD for my lights only -- hooked up to its own GFI, and then I'll run the rest of the items, heaters, fans, pumps, etc. on my DC-8 which will be on its own GFI. That way if my lights trip a GFI, everthing else will keep on ticking...
Jack