It tastes funny.
It’s perfectly safe, I drank tap water exclusively for my first three weeks here in Zaragoza (I’m someone who drinks a lot of water on a daily basis, partly because I run several times a week), and I had absolutely no problems whatsoever. It’s fine.
However…it does taste weird. I don’t know how to describe it, but it definitely is noticeable (I’ve done a blind taste test: yes, I can easily distinguish Zaragoza tap water from bottled mineral water), it tastes different from tap water at home which tastes like nothing (like distilled water, very neutral), and it tastes different from the mineral water here which is why mineral water is so popular in Spain and Europe in general: because the tap water doesn’t taste as good, that’s why.
A clarification on what “mineral water” in Europe is…
It doesn’t usually refer to Perrier, it is not carbonated…usually. This is what Americans normally think of when you say “mineral water”. Yes, mineral waters can be carbonated here, yes Perrier is one of them, yes they are sold here…however, that’s not what people mean when they say “mineral water”, or “agua mineral”, that’s not what you’ll get if you ever ask for an “agua mineral” at a bar or restaurant here. What you’ll get will be what we would call “bottled water”, that is plain, flat (non-carbonated), bottled water that came from a spring. Fiji and Evian are examples of what I’m talking about that you may be familiar with.
If you want carbonated mineral water – like Perrier – what you’ll need to specify is “agua mineral con gas” (literally “mineral water with gas”).
So do Spaniards drink the tap water?
Yes, but not if they can help it, it seems. That is, drinking solely mineral water can get expensive, depending on how much of it you’re drinking (or your guests, like at a dinner or party, are drinking), and what your budget is, but it’s not that expensive. Like almost everything else in Spain, especially food and drink, it’s really quite cheap.
For example, a 6-pack of 1.5 liter bottles (so 8 liters in total) of my preferred brand (described below) at the local grocery store (Eroski) costs €4.14 ($4.56US or £2.95GBP at this writing).
Ok, so what’s good? What should I buy if I go there?
Solan de Cabras. There are others I’ve tried, they’re fine, it’s not like this really matters that much…but that’s my preferred brand. I’ve tried others, I didn’t like them as much. This one tastes like water should: nothing. It tastes like water and nothing else.
I’ve noticed that almost always when there’s some sort of high-level political meeting on the news and there’s water on the table, it’s this particular brand (there are a lot of Spanish mineral waters and no, this one isn’t state-owned or anything).
I’ll leave it at that for now. Later I’ll probably come back and edit this a bit after I ask some friends about this topic.