What Chemicals Should I Add to My Pool for Winter? (Including Borates)

Quick answer: Before closing a pool for winter, clean and balance the water, shock it a few days ahead, then add a stain and scale preventer and a winter algaecide, with optional enzymes and borates. Add them in order, never all at once, and follow each product label.

The usual closing order

• Clean and balance first: brush, vacuum, empty baskets, and bring pH to about 7.2 to 7.6 and alkalinity to about 80 to 120 ppm
• Shock a few days before closing, after sundown, and let chlorine fall before adding algaecide
• Add a stain and scale preventer (metal sequestrant), especially with well water or high metals
• Add winter algaecide last, right before the cover goes on
• Optional: enzymes or a slow-release winterizing pill to help break down oils and organics
• Never pour antifreeze into the pool water

What borates do

• Borates buffer pH against rises, which helps control scale and keeps chlorine working better
• They act as an algaestat, meaning they help prevent algae but do not kill an existing bloom
• They do not evaporate, so levels stay fairly steady and only drop when water is removed or replaced
• The common target is 30 to 50 ppm, measured as boron, and 50 ppm should not be exceeded

How to add borates safely

• Add them to clean, clear water, not cloudy or algae-filled water
• Use a borate test kit, since standard kits do not measure it
• Add in stages and let the pump circulate between doses rather than adding the full amount at once
• Borax raises pH, so plan to lower pH afterward. Boric acid raises it much less
• Retest pH and alkalinity after circulation, because borates buffer the water and make big pH swings harder to correct

Common mistakes

• Adding chlorine shock and algaecide on the same day, which cancels the algaecide
• Closing while the water is still warm, so winter chemicals get used up quickly
• Using borates to treat an algae problem instead of preventing one
• Mixing chemicals together or skipping the label dosage

When to call a pro

Not every pool needs every winter chemical. Your climate, cover type, water source, and salt system all matter, so ask your pool professional what fits your pool.