Moved into a house 2 years ago with a circa-1970s 40k gal plaster pool. We've been using trichlor tabs and shock for chlorination , adding shock and balancing pH based on hard-to-read test strips. The pH has been fairly stable for the past 1.5 yrs and we've been attempting to maintain 3-4 ppm FC based on the test strip manufacturer recommendations.
We'd been fighting mustard algae blooms every 1-2 months and couldn't figure out why as we'd been maintaining the recommended chlorine levels. Every 1-2 months required an expensive and pain staking regiment of 32 oz of Polyquat 60 + 32 oz of clarifier followed by a vacuum to waste of the dead algae.
I stumbled onto the BBB method section of this site a week ago and immediately ditched the trichlor and shock, switching to Clorox 8.25% bleach from Costco (until I stock up at the next bleach sale). Just received my TF100 test kit and recorded a pH of 7.8, FC of 4.5ppm, CC of 0-0.5ppm, and CYA of 110-120 ppm before I ran out of sunlight. Currently bringing the FC up to 9ppm per the Pool Calculator.
Will take TA and CH levels tomorrow. Looks like a 1/2-1/3 water replacement is in my future. Hope to have this old pool in check soon.
EDIT: Today's measurements
FC = 9 ppm
pH = 8.2
CYA = 110 ppm
TA = 130
CH = 470