Closing your pool before winter is not just about putting on a cover and walking away. The water inside your pool continues to react to weather changes, organic matter, and even small chemical imbalances. Without the right pool closing chemicals, you risk algae growth, equipment damage, stained surfaces, and costly repairs when reopening in spring.
In this detailed guide, we’ll explain why closing chemicals are essential, which ones you need, and how to use them effectively. Whether you own an inground or above-ground pool, proper chemical treatment ensures crystal-clear water and protects your investment.
Why Pool Closing Chemicals Matter
Skipping chemicals when closing a pool may seem like a way to save money, but the long-term costs often outweigh the short-term savings. According to the CDC (Centers for Disease Control and Prevention), improper pool maintenance is a leading cause of recreational water illnesses and equipment breakdowns.
Key Reasons You Need Closing Chemicals
Prevent Algae Growth: Cold water slows algae, but it does not stop it completely. Without chlorine or algaecide, green, black, and mustard algae can bloom under your cover. Learn more about beating pool algae to understand why proper chemical balance is crucial.
Protect Surfaces: Balanced water prevents staining, scaling, and etching on tiles, liners, and concrete. Poor water chemistry can lead to various pool problems that are expensive to fix.
Extend Equipment Life: Corrosion from unbalanced pH or alkalinity damages pumps, heaters, and filters. Understanding proper pH levels is essential for equipment longevity.
Ensure Easier Opening: A properly closed pool requires fewer chemicals and less cleaning in spring. This makes your pool opening process much smoother and cost-effective.
Think of closing chemicals as an insurance policy for your pool water. They protect the pool all winter long, even when you’re not checking it daily.
Essential Pool Closing Chemicals You Need
Here are the main chemicals recommended by professional pool services across the United States:
1. Chlorine Shock (Oxidizer)
Purpose: Sanitizes the water by killing bacteria and organic matter.
When to Use: Add 2–3 days before covering the pool so chlorine can circulate properly.
Pro Tip: Use a non-chlorine shock if you close late in the season to avoid overstabilized water. If your pool is losing chlorine fast, you may need to address underlying issues before closing.
2. Algaecide
Purpose: Prevents algae growth during the dormant months.
When to Use: Add just before covering to provide long-term protection.
Pro Tip: Choose a polyquat 60 algaecide, which is non-foaming and long-lasting.
3. pH and Alkalinity Adjusters
Purpose: Keep pH between 7.2–7.6 and alkalinity between 80–120 ppm to prevent corrosion or scaling.
When to Use: Test and adjust levels before adding other chemicals.
Pro Tip: Add alkalinity increaser before adjusting pH for better stability. Regular pool water testing ensures you maintain proper chemical balance.
4. Calcium Hardness Increaser
Purpose: Prevents water from leaching calcium from plaster or vinyl liners, which can cause etching or damage.
Target Range: 200–400 ppm depending on pool type.
5. Pool Enzymes (Optional but Recommended)
Purpose: Break down oils, lotions, and organic contaminants that chlorine alone cannot remove.
Benefit: Reduces waterline ring and keeps water fresher for spring opening.
6. Pool Antifreeze (for Lines)
Purpose: Protects plumbing lines from freezing in colder climates.
Note: This is not poured into the pool but added to pipes after blowing them out.
Step-by-Step Guide to Using Pool Closing Chemicals
- Test the Water: Use a reliable test kit to measure pH, alkalinity, calcium hardness, and chlorine levels.
- Balance the Water: Adjust alkalinity first, then pH, then calcium hardness.
- Shock the Pool: Add chlorine shock 2–3 days before closing. Run the pump to circulate.
- Add Algaecide: Pour evenly around the perimeter before covering.
- Enzyme Treatment (optional): Add after algaecide for extra protection.
- Drain Water Levels: Lower the pool water as required for your pool type.
- Add Antifreeze: After blowing out lines, pour antifreeze into plumbing lines.
- Cover the Pool: Install your winter or safety cover securely.
Cost of Pool Closing Chemicals
The price of closing chemicals varies depending on pool size and product quality. Here’s a breakdown:
| Chemical Type | Average Cost (per season) |
|---|---|
| Chlorine Shock | $20–$50 |
| Algaecide (Polyquat 60) | $15–$40 |
| pH & Alkalinity Adjusters | $15–$35 |
| Calcium Hardness Increaser | $20–$45 |
| Enzyme Treatment (Optional) | $25–$50 |
| Pool Antifreeze | $5–$10 per gallon |
👉 On average, homeowners spend $75–$200 on closing chemicals each season, a small investment compared to the cost of draining, refilling, or repairing a neglected pool. For a detailed cost comparison, check our guide on DIY vs professional pool closing costs.
Common Mistakes to Avoid
Closing too early: Wait until water is below 65°F to avoid algae blooms. Learn more about when to close your pool in New Jersey for optimal timing.
Skipping chlorine shock: This is the #1 reason pools turn green by spring.
Overdosing algaecide: More is not better; follow label directions.
Forgetting antifreeze in cold climates: Frozen lines can burst and cost thousands to repair.
Not circulating after chemical addition: Always run the pump before shutting down.
Understanding what happens when you don’t properly maintain your pool highlights why these steps are crucial.
Winter Pool Covers and Chemicals: A Team Effort
Even with the right chemicals, your pool cover plays a huge role in protection. A winter pool cover prevents sunlight and debris from entering, while a safety cover provides accident protection as well.
For a comprehensive comparison, check out our detailed guide: Winter Pool Covers vs. Safety Covers: Which Is Better for Closing Season? This helps you decide the right cover to complement your closing chemical routine.
How Professional Services Simplify Pool Closing
DIY pool closing is possible, but many homeowners prefer professionals for peace of mind. Expert technicians not only apply the correct chemicals but also:
- Blow out and protect plumbing lines
- Inspect equipment for wear and tear
- Ensure water chemistry is perfectly balanced
- Install covers securely to withstand winter storms
Understanding why DIY pool opening often costs more than professional service also applies to closing procedures.
👉 Learn more about our pool closing services in New Jersey. We handle everything from chemicals to cover installation across Mercer County, including Hamilton Township, Plainsboro, West Windsor, East Windsor, and Robbinsville.
FAQs About Pool Closing Chemicals
1. Can I use regular chlorine instead of shock?
Regular chlorine tablets work slowly. Shock delivers a quick, powerful dose to kill bacteria before closing.
2. Do I still need algaecide if I shock the pool?
Yes. Shock kills existing bacteria and algae, but algaecide prevents new growth during the months your pool is covered.
3. What happens if I don’t balance alkalinity before closing?
Low alkalinity can corrode metal parts, while high alkalinity causes scaling and cloudy water.
4. Is pool antifreeze necessary in mild climates?
If your area never freezes, you may not need it. In colder states, it’s essential to avoid burst pipes.
5. Can I add all chemicals at once?
No. Add them in stages, starting with balancing agents, then shock, then algaecide. Mixing can reduce effectiveness.
Final Thoughts
Pool closing chemicals are not optional — they’re essential for protecting your pool during the off-season. Without them, you risk algae blooms, surface damage, and costly equipment repairs by spring. From chlorine shock to antifreeze, each chemical plays a specific role in maintaining water health and safety.
For homeowners looking to save time and stress, professional pool closing is the safest option. At Desi Boys Pool Services, we provide expert seasonal care, ensuring your pool is properly balanced, protected, and ready for next year. Whether you need weekly pool maintenance, a pool opening service, or a full pool closing package, our team delivers reliable results with your pool’s health in mind.
Ready to prepare your pool for winter? Contact us today to schedule your professional pool closing service and ensure your pool is properly protected for the season ahead.
