I'm thinking... do you have a HOB filter? Sometimes bubbles build up and collect on upper level things in the water, much like that.
Often can be due to type of dechlorinator, other water additives, excess organics in the water, etc.
I get it in my blackwater tank because I use rooibos tea, I find it sometimes causes bubbles from the HOB filter.
Along the sides of my blackwater tank. Both rooibos tea for tannins and leaf litter (organics) that cause it in this tank for me.
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Also got the same when I used API water conditioner and the Nutrition AquaPlus conditioner .