Balance pH and alkalinity from a fresh test BEFORE the chlorine raise. Freeze-zone pools: lines blown and plugged is the part that protects the plumbing — chemicals protect the water.
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The closing order
1) Test and balance (pH 7.4–7.6, alkalinity 80–120 — doses from our acid and alkalinity calculators). 2) A day later, raise chlorine to ~10 ppm. 3) Add winterizing algaecide once chlorine has drifted below ~5 (high chlorine degrades algaecide). 4) Clean thoroughly, drop the water level per your cover type, blow and plug lines in freeze country (how much antifreeze), cover tight. Not sure it's time yet? Check the water temperature first.
A worked example
15,000 gallons at 3 ppm: reaching 10 ppm takes about 111 fl oz — one gallon jug — of 12.5% chlorine. Add a bottle of polyquat 60 at its winter rate two days later, and the total chemical bill for a clean spring opening is around $25. Compare that against the green opening it prevents: three-plus gallons of chlorine and a week of brushing.
Why closings fail
Three culprits, in order: closed too warm (biology kept running), cover gaps (sunlight + debris = spring soup), and unbalanced water sitting six months (scale or etching, depending which way it was off). The chemicals above are cheap; the discipline about temperature and cover fit is the actual kit.
Frequently asked questions
What chemicals do I need to close my pool?
The core kit: liquid chlorine for a final raise to ~10 ppm, balancing chemicals per your last test (pH 7.4–7.6, alkalinity 80–120), and a polyquat-type winterizing algaecide. Skip 'winter pills' and floaters that promise all-in-one — balanced water under a good cover is the actual protection.
When should I close my pool?
After water holds below ~60°F consistently — algae is nearly dormant there, so what you lock in stays clean. Closing on a warm October weekend because it's convenient is how pools open green: the water stays biologically active under the cover for weeks.
Should I shock the pool before closing?
Raise chlorine to about 10 ppm a day or two before the cover goes on — high enough to sanitize deeply, with time to drift down so it doesn't bleach the cover or liner sitting stagnant at 20 ppm.
Do I really need winter algaecide?
With cold water, a tight cover, and 10 ppm of closing chlorine, it's genuinely optional — but polyquat 60 is cheap insurance against a warm spell, and it's the one algaecide class that won't stain or foam. Follow the bottle's winterizing dose.