If you fill a glass container with tap water and let it stand for an hour or so, the sides of the container will often be covered with bubbles. That is a result of dissolved gasses in the water coming out of solution due to the lower pressure and slow warming of the water. When we add lots of tap water to our tanks, the same thing happens, and bubbles also appear on plants, hardscape, etc. But, in addition, one of the gases in the water is CO2, which the plants use, and that can give us some real pearling, which is oxygen being given off by the plant as an end product of the use of CO2 along with fertilizers by the plant. So, some of the bubbles are pearling, and some are just outgassing of the tap water.