So I recently bought my GCP and have been using it with straight tap water because I knew the water in my town was quite soft and I didn't have time to get really into the water topic just yet.
Well now I decided to dig deeper into the topic and I'm left confused as to whether or not I could use our tap water since to me the water readings from our local water authority seemed to indicate a decent water for brewing, though I'm unsure since the values in the report are German and most information about brewing water is English. I hope someone here might be able to help out, here's the water report:

The calcium, magnesium, and chloride concentrations look quite good as regards risk of scale formation or corrosion. The pH is probably OK as well, though for some reason seems just a bit more alkaline than I would have guessed given the other ionic concentrations.

Yeah, it's an odd water, with far more alkaline, carbonate hardness than calcium/magnesium. The 7 German degrees of total hardness calculates to 125 mg/L equivalent. That's nice for the taste; but to prevent scaling, dilute it 50/50 with distilled or RO water to get the hardness down below 75 mg/L

I agree with Jim that it would be scale prone if not softened or diluted. But I don't find it odd in terms of alkalinity versus total hardness.

Doing conversions into conventional 'as CaCO3' chemical equivalence units:

Gesamtharte 7.0 °dH = 125 mg/L as CaCO3 total hardness
Karbonatharte 6.0 °dH = 107 mg/L as CaCO3 carbonate hardness (alkalinity)
Calcium 33.6 mg.L (as ion) = 85.3 mg/L as CaCO3 calcium hardness
Magnesium 10.1 mg/L (as ion) = 41.6 mg/L as CaCO3 magnesium hardness

The chloride at 14.5 mg/L is low enough to live with -- no reason to resort to RO in order to eliminate chloride, and the alkalinity is high enough that you could consider using a decarbonizing softener. (Decarbonizing, aka hydrogen exchange or WAC resin softening, like you get in many popular filters like the BWT Bestmax, Mavea/Brita Quell, and Claris Everpure, can cause corrosion concerns if the carbonate hardness is not high and you have significant chloride ion in the water.)

P.S.
Sometimes we do see fairly odd tap water where the alkalinity is higher than the hardness, which can be caused by a water utility using soda ash treatment or even cation exchange to reduce hardness, but that's not the case here. This water has more hardness than alkalinity, and so the actual carbonate hardness is equal to the alkalinity.

Maybe my chemistry is off -- but shouldn't you expect the anion hardness (alkalinity) and cation hardness (calcium, magnesium) to be the the same if you have rain water running over limestone?

another_jim wrote:Maybe my chemistry is off -- but shouldn't you expect the anion hardness (alkalinity) and cation hardness (calcium, magnesium) to be the the same if you have rain water running over limestone?

Yes, exactly*. And what you almost always see in tap water reports is an alkalinity slightly lower than the hardness, due mostly to hardness cations associated with chloride or sulfate. That's what we have in this water report. You rarely see natural water where the alkalinity exceeds the hardness. But it's what you get if you use a conventional softener or a soda ash treatment that replaces hardness cations with sodium.

I'm not clear about what you meant earlier where you said:

another_jim wrote:Yeah, it's an odd water, with far more alkaline, carbonate hardness than calcium/magnesium.

Because this water looks typical to me in that it has slightly less alkalinity than total hardness.

PS
This is key to understanding why some people sell a simple alkalinity test kit and call it KH, or carbonate hardness, or temporary hardness. As long as the carbonate/bicarbonate alkalinity is equal to or lower than the hardness, which is almost always the case with natural water, you can say that your actual carbonate hardness is equal to your alkalinity. In cases like softened water or recipe water where your alkalinity may be much higher than your hardness, then carbonate hardness, strictly defined, would be equal to your total hardness.

* [Edit addition] Well, if we want to quibble about acid rain, then it would not be perfectly exactly. The sulfuric acid in acid rain would tend to dissolve some of the limestone as calcium sulfate rather than calcium bicarbonate, and to the extent that happens you do expect lower alkalinity than hardness.

Ah, I was reading it wrong. "Gesamthaerte" literally translated means total hardness. So I thought that implied that if the alkalinity is 6 degrees, the remaining cation hardness is 1 degree. But that doesn't jibe with the 44 mg/L calcium plus magnesium. I guess "Gesamthaerte" means the same as the old "permanent hardness," so that this is completely standard issue hard water.

In any case, for the OP: it needs a softening cartridge or dilution.

Me too...I skipped over the total hardness section of the report and just went from the Ca/Mg concentration levels to the pH report. Now it makes sense. Thanks for the explanation, homeburrero!