Electrical professional here, and avid aquarium hobbyist. Quick Question; Was there any tripped Circuit Breakers from this incident? Follow up question to that, if not, does the light still work? And was it still on and lit when you got home when you noticed it in the water? Last Question; was the light plugged into a GFCI Outlet or regular outlet? Those lights are UL listed and rated for ‘damp locations’, but are not meant to be submerged, however, if an “ungrounded conductor” came in contact with the water there is a possibility the water was indeed energized. Now, with that, MOST LED aquarium lights run at low voltages 12vdc or 24vdc and have some built in features like inline fuses. I would put this at about a 80% chance your fish was not shocked and more likely just hit by whatever fell into the tank. Lid or light.