I know that trichlor lowers PH and contains CYA. When I started up the pool last week - I filled a floater with 9 1" Trichlor tabs. Since the pool is plaster - it will help keep my PH down and add CYA.

Test results yesterday morning (dialed back the floater 1/2 way):

FC=5 (OTO)
TC=5
PH=7.5
Alk=90

and this morning I get these numbers:

FC=1.5 (FAS-DPD)
TC=1.7
PH=7.2
Alk=70

Is the PH drop more than likely linked to the trichlor in the floater - even though I reduced the flow on it?

Also at what point should I start testing for CYA? My chlorine level has remained (before I cut back the floater) between 3ppm-5ppm in the morning and 0-1ppm after a day in the sun. Should I wait another week?