Hasnt "soften water" just had the "hardness" removed by ion exchange?

Exchanging calcium/carbonate ions (good for reef-bad for pipes, water heaters, coffee-makers and showerheads) with the ions of salts that wont clog your plumbing.

The problem with reef tanks and water is other contaminates that I dont think water softeners remove-such as heavy metals, chlorine and organics.

In fact, I would like a little hardness in my water and many people "remineralize" RO/DI water to get the hardness back.

If I couldn`t use good RO water, I would just use de-chlorinatied tap water.