Brian in contact with what?
What in that room is both grounded and within reach?
Without seeing the room, I don't know. One possibility is a metal stand for the aquarium with a power strip (with a metal case) sitting on it, plugged into a grounded outlet. (A power strip isn't that unlikely, since a basic setup (light, air pump, and heater) requires three cords.) Possibly through the floor, depending on footwear, flooring, dampness, etc.
I don't know about the fish. How much current would go through the fish and how much would go around it?
Better designs of aquarium lights would help (and there probably are some better ones available). Make it so it fits more securely on the top. A cat shouldn't be able to knock it off.
