My outlets are GFCI protected and the OP stated theirs was as well but it didn't trip. I guess what I was looking to do was conduct a random check on the tank for stray voltage (a sign of something going bad) based upon a normal/base/known operating stray voltage.
A GFCI wont trip until some of the current within the system 'disappears'. IE; The OP caused this to happen by GETTING shocked, he diverted the current. The GFCI either didn't trip cause its faulty, or didn't trip because the period of time that the current was diverted for, or the amount of current diverted, wasn't sufficient to trip.
I recently built a GFCI box for my reef system, and I'd stand by it. Its not a smart idea to start sticking meters into the tank without a strong understanding of what you are doing.