Chemistry - How to correct the pH and composition of bottled water?
Long time ago, I supervised an undergraduate on a summer tea project (for fun). In South Asia there is a natural strawberry colored tea made from slightly matured green tea. You boil the leaves in water with a small amount of sodium bicarbonate. Ice cool it, and boil again and aerate it with a ladle. The tea becomes reddish on aeration and after adding milk, it becomes pinkish. So I can recall your experiences because I tried this recipe in another country. No matter what I did, the right color would not develop. It remained murky brown. It turned out that the chlorinated water was ruining the natural colors, it was particularly destroying the reddish pigment. Passing the tap through a carbon filter+ion-exchanger did not work as calcium also seemed important for the right color.
Finally a mineral water bottle worked and the undergrad saved a project for his poster presentation.
Anyway, it seems your tap water is of bad quality or it is hard. This tea "scum" are Ca/Mg salts. You guessed it right. My suggestion is do not add any acid such as citric acid. Tea is not meant to be made in the presence of citrate salts especially if you add milk to it. I do not know what the taste be like. Most likely horrible!
I do not know how easy it is find potable distilled water in stores. Ask the manufacturer if it is fit for human consumption. Distilled water is not ultrapure water as it may have organic impurities or volatile impurities. It is only free from water soluble salts. All these tea scum problems should vanish.