as long as they are frequent small bubbles, and not steady streams of almost microscopic bubbles.
the former is pearling, and means the water has reached O2
saturation so the plants respirining oxygen must stay in gas form.
The latter is streaming, and indicates plant trauma/tissue damage, and they are 'bleeding' oxygen into the water.
Its most likely pearling though, since you indicate it takes until evening to occur...O2
saturation has to build up during the day before pearling takes place.