Thanks to everyone for your answers, as always things are more complicated than they seem!
Thank you for your kind welcoming!
Yes, it is a single layer of steel
Luckily I didn't add it to all the bottles, but I'd say to a third of them.
However as I said at the time of bottling this colour change\reaction wasn't that obvious, it was only when I cleaned it hours later. To play it safe I could dump them and change my procedure next time (using glass I guess) even though that would be a shame
Unfortunately in Italy where I'm from it seems almost impossible to find drinkable distilled water. The only type you can find is the one used for ironing or for such appliances and it specifically says it's not for consumption. I've been looking into it because I'm also passionate about tea and coffee and many people are experimenting with the so called "water recipes", starting from distilled and adding minerals in specific ampunts to create a good water profile.
And yes, my water has chlorine, but the smell seems to go away from the bottle in a little over a day, the water I used was decanted for a while, not straight from the tap, so I would say very little was left
Thanks, this is interesting information!