That was my first instinct but I have been reading a lot about stray current and grounding probes. Some say you need to isolate the piece of equipment that is responsible for the stray current and fix or replace it. They say that a probe just guarentees that current will be directed through the tank and it's not a good thing.Might want to get a grounding probe. Here is link with a member having a similar issue: http://reef2reef.com/threads/electrical-shock.226055/#post-2619554
I did install a new GFCI in the outlet the display tank is plugged in to as well so I am protected from full out electrocution until I figure out which piece of equipment is the bad actor. I read you can use a voltage meter and put 1 prong in the water and the other prong in the ground of the outlet while 1 piece of equipment is running. If it registers current then it's the bad actor.