[Hoppy]

ADA Aquasoil and CO2 are not going to kill plants. They are both very beneficial to plants. With the high KH you have your pH is not going to drop down enough to consider the water at all acidic. Something else is causing your plants to die, possibly inadequate light or too cold water. In other words we need a lot more information: lights, rocks, what plants, what water temperature, etc.

 

[Hoppy]

Softened water either has a lot of sodium ions in it, or a lot of potassium ions, depending on what salt is used in the softener. Almost all homes with softeners use unsoftened water for hose bibbs, so plants can be watered without harm.

A couple of years ago, perhaps less than that, one of our experts here stated that softened water, even that using potassium chloride as the salt, is not going to work for aquatic plants at all. That was in response to the standard advice of switching from sodium chloride to potassium chloride so the water would be usable.

I'm not familiar with how a water company would treat their water when it is extremely hard. I suspect a careful Google search would tell us that.

EDIT: I found something: http://www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/lime_softening_DWFSOM41.pdf The process for removing Calcium and Magnesium ions is to raise the water pH to around 10-11, which causes both of those ions to be precipitated out. It is easy to see that some mistake in applying this process could produce water not suitable for aquatic plants.