Regardless if it harms the stuff in the tank or not. It shouldn't give any tingle sensation if you touch it. I'd say it might be a little defective and would have removed it like you did. Now, if it harms stuff in the tank. since it's not submerged in the water I don't think the there would be any voltage in the tank. I'm not a electrician or anything but does the plug have a ground prong on it? Also, with the GFCI have you tested it recently. I've seen this go bad and not work. I have a little tester that plugs into the recep and when you push the little button it trips the GFCI. Similar to what happens when you press the test button on the GFCI itself. I like using this method since it's not using a internal device to the GFCI that I feel could provide a false test. But it sounds like there might be a bare wire in the light fixture some place.
Hopefully someone with some more electrical background will chime in.