Clear As Glass Ice Cubes?

Flowing water does make a great clear ice cube as well as the other tips offered here. The reason boiled water and distilled water make a clearer ice cube is because they lack the air that's entrained with most tap water. It's the air in the water and any other impurities that freeze after the pure water around the sides freeze. In the old days of ice making plants, there was a device known as a "core sucker" that sucked out the center of the ice block as the outer shell was freezing and replaced it with fresh water to make a clearer block of ice. Pure water freezes first and then the impure water with air and minerals freezes last and that part of the block/cube is cloudy. Nothing in it that will hurt you but it looks better.