This is a bit off topic, but is anyone au fait with the Leuze in Bad Cannstatt? For the second time I swam there on January 1, and I spent some time in the 20 degree outside pool with water from two springs. I didn’t write down the names of the five minerals there. It’s far too cold for me, but once you have swum nearly a length, the tingling in the water makes you feel warm all over, so you can keep swimming comfortably even though it’s actually cold. What’s more, the temperature keeps small children away. Lots of carbon but no chlorine.
There’s a detailed review online, but the reviewer didn’t try this particular pool because it was so cold. I haven’t yet found anything describing this effect.