Removing calcium and magnesium?

[chem geek]

A water softener is just an ion exchange material (typically in the form of beads) so it will accept certain ions, calcium and magnesium which form the basis of "hard" water, and release sodium and/or potassium ions in their place. If this is all that it does, then it should lower CH, but everything else should stay the same (pH, TA, etc.). TDS (in ppm) would actually go down if Calcium were traded with Sodium (which has a lower molecular weight and is singly charged).

This is a good idea that I didn't think of. I just don't know how economical it would be for volumes of water in a pool. When a water softener cartridge gets filled up with magnesium and calcium, you replace [EDIT] or regenerate [END-EDIT] the cartridge, so they are typically rated in terms of how many gallons of water at a certain hardness they can filter. It's the best idea proposed so far, however, for reducing calcium and magnesium and I'm really glad dannyboy thought of it. Clearly for fill water, it is very economical -- I'm just not sure how much it would cost to filter your entire pool if you've already got high CH in it.

Richard