Tap water does conduct electricity because there are still ions present that allow it to do so. Distilled water, however, cannot conduct electricity because all ions have been removed. The ions present in tap water allowing for conduction are magnesium and calcium salts.
'Salts' is the keyword in this statement—sea water also conducts electricity very easily, as does pool water because of chlorine which is also a salt. Although all of these 'types' of water are great conductors, PURE water is actually a great insulator.
Pure water is an excellent insulator. Plain water is a good conductor. Plain water is the water that comes out of the tap or out of the sky like rain. Plain water contains different types of dissolved minerals. These include calcium, magnesium, iron, and sodium.
For electricity to travel through liquid, a movement of charge must take place through the fluid. In tap, rain and sea water there are many impurities such as salt, calcium, and magnesium. When charged ions move through water, electricity can flow through the liquid. Distilled water, however, is unable to conduct electricity.