9 Essential Pool Closing Chemicals and Accessories

As the autumn air cools and leaves begin to drop, a backyard pool shifts from a summer oasis to a structure vulnerable to the elements. Failing to prepare this valuable asset for freezing temperatures can result in cracked plumbing, torn liners, and a green swamp come spring. Equipping a pool with the proper closing chemicals and physical barriers ensures the water remains clear and the structural components remain intact through the harshest winter months.

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Why Proper Winterization Prevents Costly Spring Repairs

Water behaves in brutal ways when temperatures drop below freezing. As water turns to ice, it expands by approximately nine percent, exerting immense outward pressure on everything containing it. Without proper preparation, this expansion easily ruptures underground PVC plumbing pipes, cracks heavy plastic skimmer wells, and splits expensive filtration equipment. A single cracked pipe buried beneath a concrete pool deck can turn into a multi-thousand-dollar excavation and repair project by the time swim season returns.

Beyond structural damage, winterization protects the cosmetic and chemical integrity of the pool. Algae does not simply disappear because the pool is closed; it opportunisticly blooms during mild autumn and early spring days when the cover is on but the water is untreated. Failing to sanitize and balance the water before closing leads to deep staining on plaster and vinyl surfaces that requires acid washing or liner replacement to fix. Investing a small amount of time and money in high-quality closing gear is the only way to avoid these nightmare scenarios.

Winter Chemical Kit – InTheSwim Ultimate Closing Kit

A comprehensive winter chemical kit takes the guesswork out of the closing process by bundling critical off-season treatments into one pre-measured package. Rather than buying individual active agents and hoping the ratios are correct, a unified kit ensures that the pool receives a balanced dose of enzymes, stain preventers, and slow-release sanitizers. This coordinated chemical attack keeps the water clear during the long, dormant winter months.

The InTheSwim Ultimate Closing Kit stands out for its high-concentration formulas and the inclusion of a unique winterizing carbon block floater. This kit includes a powerful natural enzyme that digests organic contaminants, a metal rust and scale preventer, and a slow-releasing, non-chlorine sanitizer floater that keeps working even under a frozen cover.

• Sizing Options: Kits are available for pools up to 10,000, 20,000, and 35,000 gallons.
• Key Ingredients: Natural enzymes, chelating agents for metal control, and non-chlorine active sanitizers.
• Compatibility: Safe for all pool types, including vinyl liner, fiberglass, and traditional plaster.

Before deploying this kit, the pool volume must be calculated accurately to prevent under-treating the water. The winter floater must be weighted and secured in the center of the pool; allowing it to drift and rest directly against a vinyl liner can cause localized bleaching or damage to the material. This kit is ideal for backyard pool owners looking for an efficient, all-in-one chemical regimen, but it is not necessary for those who prefer to source bulk individual chemicals or run specialized chemical-free copper sanitization systems.

Pool Shock – Doheny’s Super Chlorine Shock

Before sealing a pool for the season, a massive sanitizing dose of chlorine shock is required to oxidize organic matter and kill lingering microscopic pathogens. This process ensures the winterizing chemicals do not waste their potency fighting existing summer leftovers. Without this initial purge, any organic contaminants left in the water will consume the winter algaecides within the first few weeks, leaving the pool defenseless.

Doheny’s Super Chlorine Shock utilizes a highly concentrated formula containing 68% calcium hypochlorite to deliver a rapid, hard-hitting sanitizing punch. This professional-grade strength quickly breaks down chloramines, cosmetic oils, and microscopic organic debris, leaving the water sterile before the cover goes on.

• Active Ingredient: 68% Calcium Hypochlorite.
• Form Factor: Easy-dissolving granular powder.
• Recommended Dosage: 1 pound per 10,000 gallons for a standard winterizing dose.

Because calcium hypochlorite is a highly concentrated granular product, it must be completely dissolved in a bucket of warm pool water before application. Pouring undissolved granules directly into a vinyl-lined pool can cause permanent bleaching and weaken the structural integrity of the liner. This shock is the perfect choice for traditional chlorine and saltwater pools that require a clean slate before winter, but it is not suitable for pools utilizing alternative sanitizers like biguanide, which react negatively with chlorine.

Winter Algaecide – BioGuard Algae All 60 Preventative

A dedicated winter algaecide acts as the ultimate insurance policy against late-winter and early-spring algae blooms. Even when water temperatures drop, algae spores can remain dormant and then rapidly multiply during unseasonably warm winter weeks. A high-quality algaecide remains active in cold water, disrupting the cellular functions of these spores before they can take root on pool surfaces.

BioGuard Algae All 60 Preventative is a highly concentrated, non-foaming formula utilizing 60% polyquat active ingredients. Unlike cheaper algaecides that contain high concentrations of copper, this polyquat formula will not stain plaster walls or turn blond hair green when the pool reopens in the spring.

• Active Ingredient: 60% Polyoxyethylene (dimethyliminio) ethylene (dimethyliminio) ethylene dichloride.
• Formula Properties: Non-foaming, non-metallic, and highly stable in cold temperatures.
• Coverage: 16 ounces treats up to 25,000 gallons for seasonal winter protection.

When applying this algaecide, it must be poured evenly around the perimeter of the pool with the filtration system running for at least two hours to ensure complete dispersion. If the pool water has high metal levels, adding a metal sequestrant alongside this product is a smart precautionary measure. This product is ideal for plaster and fiberglass pools where staining is a major concern, while it is likely unnecessary for pools in deep northern climates where the water remains frozen solid for five consecutive months.

Safety Pool Cover – Loop-Loc Mesh Safety Cover

A safety cover does far more than just keep leaves out of the water; it is a critical safety device designed to support thousands of pounds of weight. This prevents children, pets, and wandering wildlife from falling into the pool during the off-season. A high-quality safety cover tensioned across the pool deck turns a backyard hazard into a secure, worry-free surface.

The Loop-Loc Mesh Safety Cover is built with ultra-strong polypropylene mesh that blocks debris while allowing rainwater and melting snow to drain straight through. This eliminates the dangerous pooling of water on top of the cover, removing the need for a continuous cover pump.

• Material Construction: Heavy-duty, UV-stabilized polypropylene mesh.
• Weight Capacity: Exceeds ASTM safety standards, capable of supporting the weight of an elephant.
• Hardware: Corrosion-resistant brass anchors and stainless steel tension springs.

Installing a safety cover requires drilling anchor holes directly into the surrounding concrete deck or installing heavy-duty wood deck anchors. This process requires a rotary hammer drill and careful measurement, presenting a moderate learning curve for DIYers. This cover is perfect for families with kids or pets and homes surrounded by heavy autumn leaf fall, but it is not suitable for pools without at least three feet of solid concrete or wood decking around the perimeter to secure the anchors.

Pool Cover Pump – Wayne WAPC250 Water Removal Pump

If a solid winter cover or traditional tarp is used instead of a mesh safety cover, rainwater and melted snow will inevitably accumulate on top. This heavy, standing water puts massive strain on the cover material and can pull the cover directly into the pool. A dependable cover pump automatically detects this accumulation and clears it off before damage occurs.

The Wayne WAPC250 Water Removal Pump features automatic freeze-protection technology that prevents the pump from sustaining damage when temperatures plunge. Its smart sensing switch activates the pump when water reaches a depth of two inches and shuts it off automatically once the surface is clear.

• Flow Rate: Moves up to 3,000 gallons of water per hour.
• Activation System: Intelligent water-sensing technology with no mechanical float switches to get stuck.
• Debris Protection: Large, stable base design that filters out leaves and twigs to prevent clogging.

To ensure trouble-free operation, the pump must be paired with a heavy-duty, outdoor-rated extension cord and a clear garden hose that routes water away from the pool deck. The intake screen should be checked periodically, especially during heavy leaf drops, to prevent blockages that can burn out the motor. This pump is a vital tool for anyone utilizing solid safety covers or standard vinyl winter tarps, but it is entirely unnecessary for pools fitted with free-draining mesh safety covers.

Pool Air Pillow – Robelle Ice Equalizer Pillow

Above-ground pools face a unique threat during freezing weather: inward crushing forces. As the surface water freezes into a solid sheet of ice, it expands outward, putting immense pressure on the pool walls and the skimmer assembly. An air pillow placed beneath the winter cover creates an artificial void in the ice sheet, forcing the ice to expand inward against the pillow rather than outward against the pool walls.

The Robelle Ice Equalizer Pillow is manufactured from heavy-duty, 16-gauge vinyl designed to withstand extreme cold without puncturing or losing air. Its reinforced corner grommets allow for easy anchoring to the pool sides, keeping the pillow perfectly centered in the middle of the pool.

• Material Thickness: 16-Gauge heavy-duty puncture-resistant vinyl.
• Inflation Valve: Leak-proof safety valve designed for cold-weather durability.
• Dimensions: Available in standard 4×4, 4×8, and 4×15-foot configurations.

The most critical step when using an air pillow is to only inflate it to approximately 60% to 70% of its maximum capacity. Over-inflating the pillow leaves no room for compression, causing it to pop the moment the expanding surface ice begins to press against it. This simple accessory is an absolute lifector for above-ground pool owners, but it is not necessary for in-ground pools where the earth protects the pool walls from lateral ice expansion.

Skimmer Guard – Gizzmo Ultra Freeze Protection Plug

The skimmer is one of the most vulnerable plastic components on any pool because it sits right at the water line where ice forms first. If water freezes inside the skimmer throat, the expansion will easily crack the plastic housing, resulting in a costly and invasive repair project in the spring. A skimmer guard absorbs this freeze expansion while simultaneously sealing the plumbing line below.

The Gizzmo Ultra Freeze Protection Plug features a patented collapsible, hollow tube design that safely implodes under the pressure of freezing ice, protecting the outer skimmer walls. The bottom of the Gizzmo features integrated threads wrapped in heavy rubber to seal the pipe leading to the pump.

• Length: 16 inches for standard skimmers, or 21 inches (Ultra) for deep-well skimmers.
• Thread Sizes: 1.5-inch and 2-inch tapered pipe threads.
• Material: Heavy-duty, high-density polyethylene.

Before screwing the Gizzmo into place, Teflon tape should be applied to the threads to ensure a completely watertight seal. Additionally, the pool plumbing lines must be blown out with air before tightening the plug to prevent trapping water inside the pipe. This accessory is a must-have for in-ground pools located in regions that experience hard, prolonged winter freezes, but it is not needed for pools that are completely drained below the skimmer line for the winter.

Expansion Plug – Anderson Winterizing Expansion Plug

Once the underground plumbing lines have been blown clear of water using an air compressor or blower, they must be sealed off to prevent water from creeping back in. Expansion plugs are inserted into the return lines, main drains, and cleaner lines to create a reliable, airtight barrier. A failure in any one of these plugs can allow water to enter the lines, freeze, and rupture the pipes.

The Anderson Winterizing Expansion Plug utilizes high-quality, marine-grade stainless steel hardware that resists corrosion even when submerged in chemically treated pool water for months. Its heavy-duty nylon wing nut makes it easy to tighten the rubber expansion ring hand-tight without stripping the threads.

• Material Composition: Natural industrial-grade rubber with stainless steel bolts and nylon wing nuts.
• Size Range: Available in sizes #00 (for small lines) up to #14 (for large intake lines).
• Seal Type: Dual-ribbed rubber design for maximum sealing surface area.

It is critical to measure the exact inside diameter of the return jet threads before purchasing, as standard return lines typically require a size #8, #9, or #10 plug depending on the brand of fittings. The rubber rings should be inspected annually for dry rot or cracking, and replaced immediately if they show signs of degradation. These plugs are essential for any in-ground pool owner who blows out their lines for winter, but they are unnecessary for soft-sided above-ground pools that are taken down and stored indoors.

Pool Cover Weight – Swimline Double Water Tube

For pools utilizing a traditional vinyl tarp winter cover without a safety anchoring system, wind is the ultimate enemy. A strong gust of wind can easily get beneath a loose cover, billowing it upward and dumping dirty leaves and debris directly into the pool. Heavy, water-filled tubes placed around the perimeter of the pool hold the cover firmly against the deck.

The Swimline Double Water Tube is engineered with a dual-chamber design that prevents the tube from rolling off the pool deck or shifting during high winds. Constructed from heavy-duty, UV-resistant vinyl, these tubes feature leak-proof valves that seal tightly to prevent water loss over the winter.

• Chamber Configuration: Double-chamber design prevents rolling and shifting.
• Material Thickness: 18-gauge heavy-duty vinyl.
• Length Options: Standard 8-foot and 10-foot lengths for easy perimeter placement.

When filling these tubes, they should only be filled to half of their maximum capacity with water. This leaves ample room for the water inside to freeze and expand without splitting the vinyl seams open. These water tubes are crucial for securing tarp covers on in-ground pools with flat concrete decks, but they are not designed for above-ground pools or pools utilizing spring-tensioned safety covers.

How to Balance Pool Chemistry Before the Freeze

Balancing the water chemistry before closing is the single most important step to prevent staining, scale buildup, and liner degradation over the winter. The process begins approximately one week before the final closing date. The pH must be adjusted to between 7.2 and 7.6, which is the ideal range to prevent metal corrosion and plaster scaling. Total alkalinity should be brought to 80 to 120 ppm, acting as a buffer to keep the pH stable during months of inactivity.

Ideal Winterizing Chemical Ranges: ┌──────────────────┬──────────────────────┐ │ Chemical Metric  │ Target Winter Range  │ ├──────────────────┼──────────────────────┤ │ pH               │ 7.2 – 7.6            │ │ Total Alkalinity │ 80 – 120 ppm         │ │ Calcium Hardness │ 200 – 400 ppm        │ │ Chlorine Shock   │ 10 – 15 ppm (Initial)│ └──────────────────┴──────────────────────┘ 

Once the pH and alkalinity are set, calcium hardness must be checked and adjusted to between 200 and 400 ppm. If the water is too soft (low calcium), it will aggressively leach calcium directly out of plaster walls or tile grout, causing permanent pitting and structural weakening. After these levels are balanced, the pool should be shocked and allowed to circulate for a full 24 hours before any winter algaecides or enzyme treatments are added, as high chlorine levels can deactivate these specialized winter chemicals.

Finally, the filtration system should run continuously for at least 12 to 24 hours before the cover is put on. This ensures all the chemicals are thoroughly mixed throughout the entire body of water, including the deep end. Skipping this step can leave localized pockets of untreated water where algae can easily take hold during the off-season.

Mid-Winter Maintenance Tasks to Protect Your Investment

Many pool owners make the mistake of assuming that once the cover is on, the pool can be completely forgotten until spring. In reality, a few simple mid-winter checks can prevent catastrophic failures during severe weather events. After heavy snowfalls, the tension springs on mesh safety covers should be inspected to ensure they are not bottomed out or slipping off their anchors. For solid covers, any accumulated water should be pumped off immediately to prevent the massive weight from pulling the deck anchors loose.

It is also wise to pull back a corner of the cover during unseasonably warm winter spells to check the water level and clarity. If the water level has dropped significantly, it could indicate a slow leak in the liner or plumbing lines, which must be addressed before the ground thaws. If the water looks cloudy or green during a mid-winter thaw, adding a liquid algaecide directly under the cover can save days of heavy scrubbing and chemical usage in the spring.

Finally, check the status of the freeze-protection accessories. Ensure that air pillows in above-ground pools remain centered and have not lost significant air pressure. For in-ground pools, check that the skimmer guards and expansion plugs remain tightly sealed and have not been dislodged by shifting surface ice. Taking ten minutes to perform these checks once a month keeps the pool safe through the darkest days of winter.

Summary

Taking the time to close a backyard pool using high-quality chemicals and rugged physical barriers is the best way to guarantee a stress-free spring opening. By investing in reliable winterization gear and maintaining proper water balance before the hard freeze, pool owners can protect their investments from expensive structural damage and unsightly algae blooms. When the warm weather finally returns, removing the cover to find crystal-clear water makes every bit of winter preparation entirely worth the effort.