Fun fact to know and tell-->
Guess what is inside a water softener?
Answer: Cation DI resin.

Remember that there are many many different kinds of DI resin. What is typically called a softening resin is a cation resin that can be recharged by exposing it to high levels of sodium. That's why you have that small tank full of salt pellets nest to your water softener.

The post above was exactly right - the softening resin removes calcium and magnesium (the two most common ions that make water "hard," and exchanges them for sodium ions.

If you have a water softener, by all means provide the softened water to your RODI system.

Russ