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Old 04-28-2005, 10:43 PM   #1
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How does salt make water "hard"?

In aquarium texts when reading specs for livebearers I often see something about how xyz species prefers hard water, add xyz amount of salt. Usually for mollies, I guess.

I don't understand how salt makes water hard. I have a water softener for my house, and it uses salt to make the water soft. So I'm seeing a contradiction there.

?
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Old 04-29-2005, 01:49 AM   #2
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Depends on how you define "hard".

The typical use of "hard water" means water high in Ca & Mg ..... you water softener exchanges the Ca & Mg for Na (or K) .... since the water now have no Ca or Mg, it is called soft.

However, fish (& other living things) don't see thing that way. They see total osmotic (or oncotic) pressure - which is a measure of total amount of salts dissolved in water. If you have 100 meq of Ca in the water (say) .... that's hard water .... the fish will see oncotic pressure of 100. Exchanged that with 100 meq of Na (what your water softener does), fish still see oncotic pressure of 100 - no difference!

So to be precise - adding (common) salt won't make water hard - it just raises the osmotic pressure (but the fish won't care!). Now if you add a salt like CaCl2, you WOULD raise both the oncotic pressure and the hardness.
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Old 04-29-2005, 09:19 PM   #3
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I understand what you're saying... but I'm still as confused as ever. This might be more of a question for a water softener forum, but how is replacing Ca and Mg with Na making water "soft?" And I can feel the difference in the shower - so what is the change that makes that difference?

And now can you answer this: I often see members around here recommending adding salt to an aquarium, usually as a preventative measure or treatment for illness. However, at the same time I see members saying do NOT use water that has been through a water softener. Wouldn't this be a good thing, since it has a trace amount of salt in it?
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Old 04-29-2005, 10:21 PM   #4
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Ca and Mg ions are bivalent. That is they have two + charges. Na ions are mono valent so they have one + charge. That goes some way to explain it. These charges readily bind to soap making it hard to have a shower.

Ca and Mg are minerals required for life. Fish which live in waters of high mineral content often need those minerals in the water to do well. Some aquarium salts will have Ca and Mg ions.

Na+ ions do not bond to soap n the same way. That is why soap lathers up well in softened water. Na+ and K+ are required to a lesser extent and are toxic at higher concentrations.... Ironic that K+ ions are integral to nerve function, but at high concentrations it causes heart failure. KCl is one of the ingreients in the lethal injection.

Sorry about the mini chemistry disertation.
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Old 04-30-2005, 12:10 PM   #5
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"People" really only care about water hardness because Ca binds to soap - making it less effective & forming soap scum. Since Na does not do that, the water softener simply exchange the Ca with Na & they call that soft water.

Although the softener removed Ca & exchange it for Na, it does nothing to the negative ions (like HCO3). So the water is still loaded with ions after treatment --- Osmolarity actually is increased as you need 2 Na to exchange for 1 Ca.

As I said before, aquatic life is concerned with osmolarity, & less so with the actual composition of the ions (within reason! ). So when we say a fish prefers hard or soft water, we really mean it prefer higher or lower osmolarity.

Personally, I think you can use water softerner water for fish, as long as you know its composition and compensate for it. In addition to lack of Ca & Mg - which fish & esp. inverts need, other trace elements are removed & that might be important. Another problem is that people think the treated water is same as R/O or "pure water", and the water chemistry will get really out of wack if you add stuff based on that erronous assumption.
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Old 04-30-2005, 08:45 PM   #6
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Ok, so if I am reading this right, water that has been put through a water softener is actually harder water from a fish's perspective because the osmolarity is increased, correct?
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Old 05-01-2005, 02:21 AM   #7
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It depends...... Fish that live in estuarine systems cope with Na ions in the water very well.
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Old 05-02-2005, 10:12 PM   #8
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Brackish fish (or SW fish) for that mater, can cope with high Na levels - but the water however, has higher osmolarity with higher Na concentration, independent of the fish. You can think of this as equal to Hard water from the fish's perspective .... but better not call salt water hard water or we'll all get confused!
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