The chlorine that is working right now — it kills germs and algae.
pH
How acidic the water is. Right, chlorine works and eyes do not sting; wrong, it damages the pool.
Alkalinity
ppm
Total alkalinity holds pH steady, so it does not swing up and down.
Calcium
ppm
Calcium hardness. Too little eats plaster and heaters; too much leaves white scale.
Stabilizer
ppm
Cyanuric acid (CYA). It shields chlorine from the sun, so outdoor pools need some.
Type the size of your pool and at least one test result.
How this is worked out
Each reading is compared with its good range. Anything outside it gets a step, and the steps come in a safe order: chlorine first, then alkalinity, then pH, then the slow ones.
pH is the tricky one: how far a dose moves it depends on your alkalinity and stabilizer, so we work it out from the water chemistry rather than a flat rule.
Acid and chlorine are always at least 30 minutes apart, and big changes are split in two with a test in between.
The product label always wins — if it says something different, follow the label.
Questions
Which reading should I fix first?
Chlorine, because it keeps swimmers safe. Then alkalinity (it steadies pH), then pH, then calcium and stabilizer, which are slow and not urgent.
Are test strips good enough?
For everyday checks, yes — but strips are coarse. That is why big doses are split: add half, test again, then add the rest.