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, cover tight.
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.