I just measured my tank voltage. It reads 36v AC. The heater, powerhead and circulation pump all contribute a portion of this voltage. I unplugged each one in turn with voltage drops after every unplugging.
Unless the voltage has a path to ground it shouldn't hurt anything, but if you stick your hand in the tank you can be completing the path and get shocked. If you are getting a reading of 36 VAC it sounds like you may have a wire crossed somewhere in your circuit feeding it through your outlet through the ground or one of your pieces of equipment has developed a short. How much does the voltage drop for each item you unplug? Normally you can trace the problem to a particular piece of equipment. Does any particular piece cause the largest drop?
Nothing specific, but I have noticed definite differences in my corals' behaviour when I have my grounding probe hooked up. I've replaced the grounding probe a couple of times now due to corrosion.
