Yes. Also, it is my understanding that calcium in water (which is a normal part due to the water taking up minerals in the ground) is making it slightly basic.

Yes. Also, it is my understanding that calcium in water (which is a normal part due to the water taking up minerals in the ground) is making it slightly basic. I don't know enough about chemistry in this area, but I think the calcium in the water is mostly calcium carbonate, which definitely is basic[1]. So it's no surprise that water taken directly from the tap is making the tea blue, but after removing the calcium carbonate (maybe as part of a removal of any salts) that the tea remains red. I think this hints that an ion exchanger[2] is being used in the "very good water filter" e40 is using. But removing calcium from water (along with other potentially beneficial minerals) is a bad idea, because we need it for building our bones etc. Hopefully those using ion exchanger are still getting enough calcium from other sources. But it's long been said that ion exchangers should not be used to "filter" water. [1] https://en.wikipedia.org/wiki/Calcium_carbonate https://en.wikipedia.org/wiki/Calcium_carbonate [2] https://en.wikipedia.org/wiki/Ion_exchanger https://en.wikipedia.org/wiki/Ion_exchanger PS. you can put some drops of lemon juice into your tea and it will shine red :) (and have a nicer taste)