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Water wise aren't you supposed to look at the residue, details on the label of the bottled water and the less residue the better.
lynx-
yes and no, minerals are great, they actually aid extraction of flavour with different minerals emphasising different flavour compounds in the form of acids.
calicium, magnesium and bicarbonates are the three to look out for, especially and calcium bicarbonate is what causes scale.
look for levels of bicarbonates that are lower than 75 ideally, not much higher than 100 ugm/l
we need some though as it acts as a buffer for the acids, taming the super bright tart flavours.calcium and magnesium can be as high as you want, but the key is balance. Its rare to find a mineral water that is super high in magnesium though.
pretty much the best value and mineral balance is waitrose essential, Tescos ashbeck spring is also very good, its just a bit low in everything so the coffee will tend to taste thin and sour.
something like Evian, which is super high in Bicarbs will tend towards flat chalky tasting coffee, with little or no acidic fruit notes.
Matisse
My best tip for the gaggia is to use bottled water not London tap. Sainsburys has some which is about 25p for two litres. It'll really reduce the amount of scale build up.