It could be CO2 from the fresh water that settles on every surface; these tend to be stationary until they are disturbed and then they release and float to the surface.
It could also be "pearling." The dissolved CO2 in the fresh water can be considerable, and photosynthesis may increase as a result. CO2 is usually the limiting factor in low-tech or natural method planted tanks. Oxygen is a by-product of photosynthesis and plants release it through the roots normally but also leaves during intense photosynthesis. These bubbles are much smaller in size, and will rise in a steady stream; if you look closely at the leaf you will see that each stream originates from one specific spot. I see this after many of the water changes, depending upon how much CO2 is dissolved in the tap water that day.