I'll throw my hat in the ring as well. It is dissolved gasses, I had the same problem years ago when I had aquariums. If you didn't have this problem last year, and the only difference is the concrete waterfall, then the waterfall could very well be the culprit. Maybe something in the concrete is dissolving in the water and causing part of the problem. After a day or 2 of not running anything, do the bubbles still accumulate?

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