I disagree with that KAhner not a bad electrician, he just didn't explain it to Smithcinema fully.
When you turn your light on, a current surge flows through your light circuit including your ballast transformer. Since the transformer is an inductive device, there is a slight lag between voltage and current when you switch the light on.
A GFCI outlet works by detecting any current flow imbalance between the hot and the neutral lines while checking for any ground current flow. If it detects either of these, it will trip.
What is probably happening is that the turn-on surge from the light spikes the ground line because of the voltage-current delay, causing the GFCI outlet to trip.
If its not your light doing it then you can check devices pretty easy.
To check, use an ohmeter to look for continuity between the ground prong and each of the other two prongs on the plug. There should be NO continuity between the ground prong and either of the other two. There should, of course, be continuity between the ground prong and any exposed metal parts or screws on the chassis of the appliance.
35gal Reef, 20gal Sump--2 years, 3 months--Shut Down
If you can't fix it with a hammer, you have an electrical problem!