Hello; Sounds like atray voltage to me as well. Back in the decades when all my tanks and hoods were metal I had occasion to experience stray voltage many times. It would hit with more pain in some spots than others, often around the nail. I always suspected that I had open cuts or such that prompted the extra pain.
The fish never seemed to be affected. I suspect that I was grounded and they were not. You probably should not stick a hand in the tank with bare feet on a damp floor.
I did install a GFCI outlet for use with my current tank setup even thou the all glass tanks and plastic hoods have proven much less likely to have the stray current.
I found lights to be the more common culprit in the past. The moisture from bubbles, splashes and such would apparently condense in and around the hood and over time leave deposits of mineral salt paths for the current to follow from the fittings.
Good luck
