[IAmBroom]

I think people might be confusing ultra-pure water with "DI water" (deionized water).

DI water is AN AMAZING SOLVENT. Waaay more powerful at dissolving low-solubility solids than tap water, or even distilled water. It's stunning IRL to see how much better it cleans optical surfaces, where single-molecule layers can visibly change the appearance.

But if left open to the atmosphere, as in these pools, it would soon be just plain old (pure) water. Ultra-clean, but not an ultra-solvent.