Im in Arizona, just south east of Phoenix( Gilbert) .
A little background
Ive had a Kinetico Model 60 on the house for a couple years. A friend gave it to me when he moved out of his house and didn't want to take it with him. I would guess that it is about 12-14 years old, but it is virtually perfect condition on the outside at least.
It is plumbed in on the outside of the house in the backyard( behind the block fence) to the main line coming into the house. ( i have a line plumbed from the untreated side that runs to the irrigation and pool which i can adjust)
Its worked pretty well as far as i can tell but in the last recent months i have noticed the the softness of the water has been less and less, and irt seems that it kind of cycles between producing fairly soft water to not soft water at all.
I suspect one tank has very inefficient media( gone bad) and the other is getting there.
I never lose water flow.
If i regenerate each tank a couple times the softness level seems to go back up for up to a couple weeks.

Ive changed the media i a softener before, along with the gravel and dont think it will be to difficult to do in this case.( i understand these dont use a gravel bed) BUT, i want to make sure that is the issue.
what else should i check? ( it seems to function fine as far as cycling and drawing brine).

If nothing of if what i check out proves to be a non issue and i want to replace the media, what is the best media to use and why?
here the TDS is 400-1200
here is a link the water quality report for the town for other information:
http://www.gilbertaz.gov/pw/pdf/WaterQualRep2011.pdf

Thanks for any direction