Well, it's not good news. And you're right, you are fighting a losing battle. Stop using the tabs, stop using the powder. Go buy some liquid chlorine a.k.a. bleach. Either the Pool Store sells it at 10-12.5% or you can buy household Clorox (or generic brand) 6%. This will not add anymore CYA to your pool.
As suspected, the CYA is off the charts which means you can't hold FC.
See the chart in my sig.....
You are going to have to do some draining of your pool water, 60% I would guess to start, will lower the CYA to a more manageable number, and the only way to do that is to drain and refill. Then recirculate, then retest.
(It took 4 partial drains and refills of my pool after I found this website to lower the CYA from over 100 down to 50.)
The pool store didn't test for CH? You need to know for plaster pools.
I would advise that you order
this kit asap, or
this one, and then set about draining/refilling. You will need to drain 60% of your pool, and even then it may take more. You should be able to drain down 1/3 safely, then refill, recirculate and retest. You may have to do a series like I did.
Hope this helps.
