Final answer:
A tank filled with water is a good conductor due to the presence of polarized water molecules and free ions, such as in tap water, which allow electricity to flow more easily.
Step-by-step explanation:
A tank filled with water is a good conductor because water molecules are polarized, meaning they have slightly positive and slightly negative ends. This characteristic makes water molecules susceptible to being attracted by a charged object. Moreover, tap water contains various dissolved ions, which are positively and negatively charged particles. These ions enhance water's ability to conduct electricity because they can move freely in the water, carrying an electric current.
Pure water, however, is an insulator and does not conduct electricity well since it lacks these free ions. It's the ions present in common tap or salty water that make it a good conductor. The presence of free ions means that electricity can flow through water more easily, which is why molten salt and salty water are conductors.