Hi mr.rgb and welcome .
Those are some odd readings you got from your aged water but regardless of your very high PH range I wouldn't mess with any chemistry and adding buffers or acids to bring it down.
I would either go with the tap I got or invest in a RO unit for the long run .
White stuff on the glass is slime not algae and it should be regularly wiped off during WCs , not only for aesthetics but to keep bad bacteria levels in check .
Cheers and good luck .