From one of the postings on RC:
Stray voltage of any magnitude is not harmful. Voltage does nothing. It is current that does something, and is what can be potentially dangerous. Without a path to ground, there is no current flow.
The voltage in the tank, can be read with the meter, because a current flows through the meter to ground, due to the voltage potential in the tank. Remove the grounded test probe, you get no reading = no current flow. Current does the work, not the voltage.
Anything under 40VAC, is considered "low voltage," as in most cases the current flow caused by this potential, would be harmless. Getting above 50VAC, as kcress indicates is "high voltage" and the potential for injury or death is present, if that voltage is given a path to ground.
Saltwater, due to its ionic nature, will always show some potential (voltage) between the water and ground. The potential may be too small to measure, or you may read 12VAC, or so. Also, inductive loads (pumps) can impress a voltage in the water--due to the magnetic fields, yet not be damaged or malfunctioning. However, if you are reading something above 40VAC, it is cause for further investigation.
To be on the safe side, I would actually remove the grounding probe from the tank and then plug in your equipment one by one and figure out what the actual cause is. All the grounding probe does is create a continuous electrical current in the tank, which isn't a good thing (imagine a continuous electrical current going through the tank and subjecting your fish and other organisms to it).