If the bubbles were forming on the bottom and sides on the liner, I would say it was an oxygen saturation, but on the surface it sounds more like some tanins in the water. Just organics caused by soil leaching into the water, most likely from your plants. It isn't harmful to fish.

Thanks Nancy!

That makes sense, I have added about ten potted plants this week.

Thanks Nancy!

That makes sense, I have added about ten potted plants this week.

I would wait a few weeks for your plants to grow and water to cycle and stabilize before adding fish :)