That could be it, it's the most obvious reason... That little paludarium next to has the same, not a speck of algae on the glass in the submerged part. And it's bursting from plant growth. The watermint and rotala indica in it go crazy. 😘 It actually needs a severe trim but yet didn't feel like it.
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But the zinc tub itself, having the most plants of all in it is growing a thick sheet of dark green aufwuchs against all the walls. And this is about 75% shielded from any light due to all the plants in it. And it starts up each year again completely clean...
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In a way this mystery is baffling me, glass stays clean the Zinc tub walls are covered with aufwuchs...
It doesn't always make any sense... Obviously it doesn't always like to grow on glass...
The 2 other tubs in the garden that also have a lot of plants but are stand-alone and both do grow a lot of hair algae... But in the tub system, I never found a single hair...