How low is the water level in your tank? And when was the last time you did maintenance on your filter? I find I get bubbles if a lot of my water evaporates or if my filter isnt clean enough to provide sufficient flow for movement on the water surface.
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i would go with a bubble nest idea , as long as you have bettas, ( Siamese fighting fish as they are called elsewhere) if not , i would scoop them out and see if they return
Tis a bit odd if you dont have any bubble nesters but it doesnt look like anything to worry about, you tried getting more surface movement to disperse the bubbles or does it just froth them up?