This sometimes occurs with hard (heavily mineralized) water. Most contaminants are acidic by nature. When the water is purified, this causes a slight increase in pH typically by 0.5 to 1 point higher. As the PH level of the purified water increases, the acidity of the water goes down and the water is no longer able to hold as many minerals in the solution. Because of this, the minerals begin to precipitate over time and depending on the mineral composition they will either sink to the bottom or float to the top. This process is known as flocculation and the precipitated minerals are usually referred to as "white floaters". This may be more noticeable after freezing or heating the water. Rest assured this is not to raise concern and the floaters are not harmful. They are minerals that were already in your water and now they are simply visible whereas they were previously invisible due to their suspension in an ionic form.