Yes, there's a round corner sponge filter that's going very lightly. They seem to enjoy it and spend time around the sponge itself.
This just happened in the past 24 hours. The filter's been in the tank for three plus weeks so I don't think the bubbles are from the filter.
Just watched them for a few seconds. They are the ones making the bubbles.
They take a bit of air and then out comes a bubble.