More than likely, it's not the light that's the problem. Any of the electrical devices in your tank could be suspect. I had this exact same issue a while back. I got a shock (enough to trip my GFCI!! - hint hint) when one hand was in the tank and the other hand touched my light fixture. Nothing like a jolt coming in one hand and leaving through the other. :eek2:
Turns out... after testing with a volt meter and not my hands... it wasn't the light fixture itself but one of the screws on the light fixture that was serving as ground. I had a powerhead (cough cough will never buy another Koralia cough cough) whose potting around the coil was split, exposing the coil to saltwater. That put 120V in my tank and when I made contact with that screw, I grounded myself giving me the jolt.
Test with a volt meter, with one probe in the water and the other touching various areas of your light fixture. Focus on the screws. Then unplug things one by one until you find the culprit.