In short, no she won't kill your fish. You two should compromise though. Just be sure she buys something 99% or better pure. Morton®Salt - Frequently Asked Questions Morton® White Crystal® Solar Salt

Basically, she won't be adding enough to alter the salinity of the water.

 

I myself would not use softener salts. While some are fine, others are not. They remove mineral hardness (caused primarily by calcium and magnesium salts) by adding other "salts" [which are not the same as sodium table salt, just so that is clear]. These other "salts" can be worse than the hardness, depending upon what they are and the fish.

Using pure water to dilute the tap water is still the safest method. The less "stuff" that goes in the water the better.

Byron.

 

A kit my indicate the remove some hardness, but the total dissolved solids stay the same. I've used water run through a softener for tanks and for the fish I was keeping I would rather of just had the regular tap water.

 

Alright, thanks guys.

And also, since plants absorb some of those minerals, would that in turn make the water noticeably softer?

 

Alright, thanks guys.

And also, since plants absorb some of those minerals, would that in turn make the water noticeably softer?

Not really. The mineral uptake by live plants is not great, and I have never heard of anyone experiencing softer water. I doubt the level would be detectable with our basic test kits. Consider that as little as half a teaspoon of Flourish Comprehensive can provide sufficient minerals--and all of them--for a week, and yet have no impact on hardness.