Aquariums typically have no route to ground meaning static and induced electrical currents can build up over time. This can be from the motor windings of pumps being energised. Generally live stock in the tank is unaffected for two reasons. The induced voltage will be quite weak with little to no amps and they have no ground to earth.
Induced voltage is the most common type of voltage that flags when hobbyists test their aquariums, its often noticed by hobbyists putting their hands in the water then touching a metal object such as a metal stand or a light chassis/mount wires that have a route to earth. This allows the small induced voltage to discharge to earth straight away and they feel a small zap, at this point they feel that they have been electrocuted by faulty equipment but this isn't the case.
To prevent induced currents or static charges a titanium ground probe or even a Titanium Schego heater, as the titanium jackets of these heaters are grounded through the plug providing a rather nice earth probe as well as a heater, can be used. This will allow any induced voltage to automatically go to ground and not build up in the tank. If there is a real voltage leak caused by the insulation break down of an electrical item the ground probe will generally cause a modern circit breaker to trip due to the short to ground being detected.