Thanks for the ideas.
Waterbear - That's what I was thinking, ammonia. But as it stands now all the ammonia Does pass thru the filter so any ammonia caused to be released from the z-lite by the low % salt solution won't really affect the overall chemistry.
gwrace - Same answer; I don't think it has much, if any effect; SWG & Zeolite
Duraleigh - That is where my post came from - 6 year old sand and we run pumps pretty much year-round here in TX - So figure 3x the time of most 'normal' run times in colder climates. Though I haven't done a complete work up on the water using my PoolSolutions kit, the fact is that you can see the dead algae come through the return nozzle when vacuuming deep end where most of the algae settled, it's a 'cloud'. Very, very fine.
Hence the original question - Time for replacement sand, or zeolite?
Remember the filter has worked fine until this winter/spring, and it appears the filtering ability now reduced. Pump pressure is fine, all is operating as normal except that it's not 'filtering' the fine, very fine particles. No sand is coming out, so no laterals are broken or anything like that. And when backwashing it does expel bugs, dirt & the like so it is filtering large particles.

Duraliegh/brock - Thanks. I am leaning that way.
I have asked several pool stores about adding DE to the sand but they never heard of it. Though they have heard that their $23 Sand Filter Cleaner works well ;-)

I have decided to replace the sand.
Will update everyone later.
(100+ today in Austin! with 'rolling blackouts', time to leave work early & swim!)
Thanks everyone.