Alkalinity & hardness? How do they work?

[yonsu]

How do alkalinity and hardness function, exactly? Weird question, I know. I've noticed that my city's water generally has very low alkalinity and moderate hardness.. yet, yesterday, I tested someone's water, and it was very soft and the alkalinity was very high.. She assured me that the container she brought the water in was rinsed out very well, and explained that the tank was new, with gravel, no salt, plastic decor, nothing unusual.. What could have caused such a massive shift?
What can be used to increase/decrease the alkalinity and hardness? Is it really that big of a deal (for most fish) as long as the levels do not fluctuate too much?

Thanks for the help!

 

[artdale]

Ok, I'll try ....
Alkalinity (I presume you're asking about carbonate alkalinity or KH) measures the quantity of carbonates and bicarbonates dissolved into the water.
Hardness (and here I guess we're talking about general hardness or GH) measures the quantity of calcium and magnesium iones dissolved.
Due to the different measure units it's possible to have a very soft water with high alkalinity (due, for example, to a high presence of baking soda, NaHCO3, that's measured by Kh tests but not by Gh tests) or hard water with low alkalinity (as for example due to the presence of magnesium and/or calcium sulfates).
I partially asked to your questions, too. You may increase kh adding baking soda, potassium carbonate, or other carbonates or bicarbonates. You may increase hardness adding magnesium and calcium as, for example, magnesium sulfate(epsom salt) or CaCl etc..
Obviously if you add, for example, calcium carbonate (CaCO3) you increase both measures as this way you add both calcium and carbonates.
To decrease levels you can change water partially with osmosis water in order to reduce salts.

PS: The use of a conductivity or tds (total dissolved solids) meter is useful in order to avoid this "paradox" that's due just to the tests used.