HELP!!

[zion-aquatics]

hi , i have recently discovered i have stray voltage in my aquarium , i know this because i can feel a sting in my paper cuts , i only feel this sting when im bare foot though , Now i went out and purchased a multimeter , and inserted the ground end into the ground plug on a outlet and then the red end in my tank , the reading is zero , but when i touch the ground end to some metal , then the red end in my tank , i get a reading , i have tried disconnecting all my equipment to see what was the problem and no matter what i disconnect it still has the shock , what i am thinking is that the shock is coming from the actual outlet. everything is hooked up to one outlet andi have one powerbar hooked to one of the plugs and that power bar runs my lighting , the other powerbar connected to the second plug runs my pumps etc , i can unplug all the lighting and feel the shock , and then unplug all the pumps and plug the lighting back in and still feel the shock , so im guessing its not the equipment but the actual plug i live in an apartment , can anyone shed some light on why this is happening to me? any help or suggestions will be very much appreciated