Well I've read through this entire thread and here i what I have to contribute. Firstly lets clarify a few things- the voltage your reading from the water to the ground on your outlet is potential voltage and even after removing all the equipment in your tank you will still read the voltage potential unless you discharge it by running some piece of metal or other conductive material from the tank water to your ground on your outlet. This will remove any stray voltage in the water until you plug something back in. Also current (amperes, or amps) is not present until you have a path back to ground which is what a ground probe will do and that's bad. Current is what kills, as little as 4 milliamps. Now as far and GFCI receptacles go they are your best safety however I would hook up at least a couple and make sure to have a powerhead hooked to each one that way if one trips while your at work you still have circulation in your tank. I'm not sure why you have such high voltage in your tank but it has to be something faulty. You will get some stray voltage potential by having equipment in the tank, any sort of motor will create more such as powerheads and pumps. The voltage these create should be minimal somewhere in the millivolt range.