Testing & Chemistry
Pool Closing Chemicals: What to Buy
Spring is decided in October. Here is the sequence, the arithmetic, and the kits that say what is in them.
We have not handled these products and we do not pretend to have. Here is what we did instead. Links to Amazon earn us a commission at no extra cost to you, and commission never decides a pick — disclosure.
| # | Product | Best for | The spec it turns on | Price |
|---|---|---|---|---|
| 1 |  Doheny's Master Pool Closing Kit, up to 35,000 gallonsTop pickEvery component is named on the listing, and at a stated 35,000 gallons it is the one kit here that covers a large inground pool in a single box. | A large-pool kit that lists its contents | Metal Out stain, rust and scale preventer, Winter Guard Control, time-released winter oxidizing tablets and oxidizing shock, plus 1 qt Natural Chemistry Metal Free and 1 qt Pool Magic Fall & Phos-Free; stated for pools up to 35,000 gallons | |
| 2 |  Doheny's Ultimate Pool Closing Kit, up to 15,000 gallonsIncludes the metal sequestrant most kits leave out, plus a time-release oxidizer that keeps working after the cover goes on. | A metal sequestrant in the box | Metal Out stain and scale preventer, Winter Guard Control, time-released winter oxidizing tablets and oxidizing shock, for up to 15,000 gallons | Check price#ad |
| 3 |  Rx Clear Pool Closing Kit, up to 20,000 gallonsThe only kit here that ships an alkalinity buffer, which is the chemistry that actually holds pH steady through a winter with no circulation. | A pH buffer in the kit | 2 x 1 lb Alkaplus pH powder, 2 x 1 lb All-in-One Shock X-Tra, 2 x 1 lb winterizing powder, for up to 20,000 gallons | |
| 4 |  Robelle Triple-Action Winter Kit, up to 30,000 gallonsTwo quarts each of algaecide and sequestrant for a 30,000 gallon pool, which is a heavier dose than the smaller kits scale to. | Larger pools in one box | 4 lb Winter Shock, 2 qt Winter Clear and 2 qt Anti-Stain Control, for up to 30,000 gallons | |
| 5 |  In The Swim Saltwater Pool Closing Kit, up to 35,000 gallonsThe same five components as the standard kit, labeled for a salt pool - where the closing job has one extra step no kit can do for you. | Salt pools | 1 qt Winterizer, 1 qt Winter Stain-Away, 3 lb chlorine-free shock-oxidizer, Winter Sorb and floater; labeled by the maker for saltwater pools, up to 35,000 gallons | |
| 6 |  In The Swim Algaecide 60 Plus, 1 quartPolyquat at 60% concentration and no copper, which is the combination that prevents algae without the stain risk. | Prevention without staining | 60% polyquaternium, non-metallic, non-foaming | |
| 7 |  Pool Mate Concentrated Metal Out, 1 quartPublishes its dose per 10,000 gallons on the listing, which most sequestrants do not, and names the eight metals it is for. | A stated dose per volume | Sequestrant dosed at 1 quart per 10,000 gallons; listed for calcium, cobalt, copper, iron, lead, manganese, nickel and silver | |
| 8 |  AquaChek Select Connect 7-Way Test Strips, complete kitThe kit that gets used. Seven readings in fifteen seconds beats a laboratory-grade kit that stays in the shed. | Weekly testing you will actually do | Seven parameters per strip: pH, total and free chlorine, bromine, alkalinity, hardness, cyanuric acid |
Why the order matters more than the brand
A closed pool has no circulation for five months. Whatever is in the water when the cover goes on stays roughly where it is: chlorine burns off, dissolved metals drop out onto the plaster, and algae waits for the first warm week. Every closing chemical exists to deal with one of those three, and each only works if the water was balanced first - which is why the sequence is the product, not the brand on the box.
The sequence
- **A week out: test and balance.** Alkalinity to 80-120 ppm first, then pH to 7.2-7.6, then calcium hardness - the order and the reasons are on how to balance pool water. Balanced water drifts far less over a winter than water closed out of range.
- **Three to five days out: shock.** About 10 ppm free chlorine, dosed by weight for your volume from the table below. Brush every surface first. Then let chlorine fall back to normal before you cover.
- **Two days out: sequester metals.** A pool on well water, or one with a heater, closes with copper and iron in solution, and five months without circulation is exactly when they stain. Dose per the label - Pool Mate states a quart per 10,000 gallons.
- **The day before: algaecide.** A polyquat, added after chlorine is back to normal so the chlorine does not simply consume it. Most kits include one; if yours does not, add one.
- **Closing day: lower the water, clear the lines, cover.** Below the skimmer for a solid cover, blow out and plug the lines in a freeze climate, and put the cover pump in the middle where the cover sags. Which cover to put on is its own decision.
The closing shock, by weight
Every kit either includes a shock or assumes you have one. A chlorine shock a few days before closing burns off the season's accumulated organics so there is less for algae to live on under the cover. The dose is the same arithmetic as any other shock: pounds of product from the percentage printed on the bag and the gallons in the pool.
| Pool volume | How much |
|---|---|
| 5,000 gal | 10 oz |
| 10,000 gal | 1.2 lb |
| 15,000 gal | 1.8 lb |
| 20,000 gal | 2.5 lb |
| 25,000 gal | 3.1 lb |
| 30,000 gal | 3.7 lb |
Some kits ship a chlorine-free oxidizer instead - In The Swim's saltwater kit states one, and Doheny's does not state the chemistry of its shock either way. A chlorine-free oxidizer is the gentler choice for a vinyl pool and it needs no fall-back wait, but it is not a substitute for a real chlorine shock on a pool that had algae this season. Do the chlorine shock first, then add the kit's oxidizer at closing.
What a closing kit actually contains
Four things in some combination: a winter algaecide, a metal and scale sequestrant, a shock or oxidizer, and often a slow-release floater or an oil-absorbing sorb. The kits differ on which of the four are present, and on whether the maker is willing to tell you.
| Kit | Sized for | Algaecide | Sequestrant | Shock or oxidizer | Extras |
|---|---|---|---|---|---|
| Doheny's Master | 35,000 gal | Winter Guard Control | Metal Out, plus 1 qt Natural Chemistry Metal Free | Oxidizing shock plus time-released winter tablets | 1 qt Pool Magic Fall & Phos-Free |
| Doheny's Ultimate | 15,000 gal | Winter Guard Control | Metal Out | Oxidizing shock plus time-release tablets | - |
| Rx Clear | 20,000 gal | None listed | None listed | 2 lb All-in-One Shock X-Tra | 2 lb alkalinity buffer, 2 lb winterizing powder |
| Robelle Triple-Action | 30,000 gal | 2 qt Winter Clear | 2 qt Anti-Stain Control | 4 lb Winter Shock, chemistry not stated | - |
| In The Swim Saltwater | 35,000 gal | 1 qt Winterizer | 1 qt Winter Stain-Away | 3 lb chlorine-free oxidizer | Winter Sorb, floater |
None of the five states an active ingredient or a percentage for its algaecide, which means none of them lets you compute a dose the way this site does for a named algaecide. You are dosing by the kit's own instructions, and the kit's gallon rating is the whole dose. Buy the size that covers your pool.
Size it to your volume, and round up
The picks
Top pick - A large-pool kit that lists its contents
Doheny's Master Pool Closing Kit, up to 35,000 gallons
Every component is named on the listing, and at a stated 35,000 gallons it is the one kit here that covers a large inground pool in a single box.
The published figure
Metal Out stain, rust and scale preventer, Winter Guard Control, time-released winter oxidizing tablets and oxidizing shock, plus 1 qt Natural Chemistry Metal Free and 1 qt Pool Magic Fall & Phos-Free; stated for pools up to 35,000 gallons
Doheny's itemizes the box: Metal Out for stain, rust and scale, Winter Guard Control as the winter algaecide, time-released winter oxidizing tablets, and an oxidizing shock. The Master version adds quart bottles of Natural Chemistry Metal Free and Pool Magic Fall & Phos-Free on top of that.
The phosphate remover is the piece almost every closing kit leaves out. Phosphate is algae food, and five months under a cover with no circulation is exactly the window in which a phosphate load becomes a green spring opening.
Time-released tablets are the difference from a kit that releases everything on day one. The point of a slow-release component is that something is still working late in the winter, long after the algaecide added in October has been consumed.
What it does well
- Every component named on the listing
- Carries a metal sequestrant and a phosphate remover, which most closing kits omit
- Time-released oxidizing tablets rather than a single day-one dose
What it costs you
- No active ingredient or percentage stated for any component
- Does not state whether the oxidizing shock is chlorine-free
- More product than a small above-ground pool needs
Skip it if your pool is well under 35,000 gallons. Doheny's Ultimate kit is the same core chemistry at 15,000, and Doheny's tells you to buy multiple kits rather than stretch one.
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Pick 2 - A metal sequestrant in the box
Doheny's Ultimate Pool Closing Kit, up to 15,000 gallons
Includes the metal sequestrant most kits leave out, plus a time-release oxidizer that keeps working after the cover goes on.
The published figure
Metal Out stain and scale preventer, Winter Guard Control, time-released winter oxidizing tablets and oxidizing shock, for up to 15,000 gallons
Doheny's lists four components: its Metal Out sequestrant, a winter algaecide, time-released oxidizing tablets and a bag of oxidizing shock. The sequestrant is the one to notice - metals that stay harmlessly dissolved in circulating water drop out and stain plaster once the pump stops for five months.
The time-release tablets are the maker's answer to the mid-winter thaw, when water warms just enough under the cover for algae to wake up. Nothing on the listing states how long they release for.
It is sized for 15,000 gallons and the maker says to buy multiples for larger pools, which is honest and mildly inconvenient.
What it does well
- Metal sequestrant included
- Time-release oxidizer for the winter thaw
- All four components named on the listing
What it costs you
- 15,000 gallon ceiling means two kits for many inground pools
- Release duration of the tablets is not stated
- No non-chlorine oxidizer option, unlike In The Swim
Skip it if your pool is over 20,000 gallons. Two of these cost more than one larger kit.
Pick 3 - A pH buffer in the kit
Rx Clear Pool Closing Kit, up to 20,000 gallons
The only kit here that ships an alkalinity buffer, which is the chemistry that actually holds pH steady through a winter with no circulation.
The published figure
2 x 1 lb Alkaplus pH powder, 2 x 1 lb All-in-One Shock X-Tra, 2 x 1 lb winterizing powder, for up to 20,000 gallons
Rx Clear lists two pounds each of pH buffer, shock and winterizing powder. The buffer is what makes this kit different - a pool closed with alkalinity in range drifts far less by spring than one closed on pH alone, which is the whole argument on our balance page.
It is all dry powder. There is no algaecide bottle and no sequestrant, so on a metal-prone pool you add your own, and on a pool that greens under the cover you add an algaecide.
20,000 gallons is a useful middle size that most kits skip - large above-ground and small inground pools land right there.
What it does well
- Alkalinity buffer included, which no other kit here has
- All dry powder, so nothing freezes in the garage
- 20,000 gallon size fits the pools the others straddle
What it costs you
- No algaecide and no sequestrant in the box
- The 'winterizing powder' is not described beyond the name
- You will be buying two more products to complete the close
Skip it if your pool greens under the cover every year. This kit has no algaecide and you need one.
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Pick 4 - Larger pools in one box
Robelle Triple-Action Winter Kit, up to 30,000 gallons
Two quarts each of algaecide and sequestrant for a 30,000 gallon pool, which is a heavier dose than the smaller kits scale to.
The published figure
4 lb Winter Shock, 2 qt Winter Clear and 2 qt Anti-Stain Control, for up to 30,000 gallons
Robelle lists four pounds of shock, two quarts of Winter Clear algaecide and two quarts of Anti-Stain sequestrant. Doubling the liquids for a 30,000 gallon pool is what you would expect, and it is what several kits fail to do when they simply relabel the 15,000 gallon box.
Nothing on the listing states whether the Winter Shock is chlorine or non-chlorine. On a vinyl liner that matters, so confirm before you use it or substitute a stated non-chlorine oxidizer.
The company has sold closing products since 1973 per the listing. That is not a specification, but it is a long time to have been in one category.
What it does well
- Liquids scaled honestly for 30,000 gallons
- Sequestrant and algaecide both included
- Complete winterizing instructions stated in the box
What it costs you
- Shock chemistry is not stated on the listing
- No sorb and no floater
- Far more product than an above-ground pool needs
Skip it if you have a vinyl liner and cannot confirm the shock chemistry. Use In The Swim's kit, which states its oxidizer is chlorine-free.
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Pick 5 - Salt pools
In The Swim Saltwater Pool Closing Kit, up to 35,000 gallons
The same five components as the standard kit, labeled for a salt pool - where the closing job has one extra step no kit can do for you.
The published figure
1 qt Winterizer, 1 qt Winter Stain-Away, 3 lb chlorine-free shock-oxidizer, Winter Sorb and floater; labeled by the maker for saltwater pools, up to 35,000 gallons
The listed contents are identical to In The Swim's standard closing kit. The saltwater label is the maker's statement that it suits a salt pool; the listing does not say what, if anything, differs in the formulation.
What a salt pool needs at closing that no kit provides: the cell comes out, gets inspected and cleaned of scale, and goes indoors, and the system is set so it cannot fire with no flow. See salt vs chlorine.
The sequestrant matters more on a salt pool than on most. A cell pushes pH up continuously all season, and high pH is exactly where dissolved metals fall out of solution.
What it does well
- Complete kit with every component itemized
- Chlorine-free oxidizer
- Sized for a large inground salt pool
What it costs you
- The listing does not say how it differs from the standard kit
- Does nothing about the cell, which is the salt-specific closing step
- Oversized for a small above-ground salt pool
Skip it if your salt pool is under 15,000 gallons. The smaller standard kit lists the same components at the right size.
Pick 6 - Prevention without staining
In The Swim Algaecide 60 Plus, 1 quart
Polyquat at 60% concentration and no copper, which is the combination that prevents algae without the stain risk.
The published figure
60% polyquaternium, non-metallic, non-foaming
Polyquat 60 is the concentration standard in this category. Products sold as 'algaecide 30' or 'algaecide 10' are the same chemistry diluted, so compare the price per unit of active ingredient rather than per bottle.
No metal means no staining, which matters on plaster and on any pool that has already had a copper problem. It is also why polyquat is the algaecide to use after you have shocked, rather than during.
Polyquat prevents. It is not a rescue product for a pool that is already green, and any label that implies otherwise is selling you the wrong step.
What it does well
- 60% active, the top of the retail concentration range
- No copper, so no staining and no green-hair complaints
- Non-foaming, unlike the cheaper quat algaecides
What it costs you
- Prevention only — will not clear an existing bloom on its own
- Chlorine consumes it, so a heavy shock burns off what you just added
Skip it if your pool is green right now. Shock first, vacuum the dead algae, then dose polyquat to stop it coming back.
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Pick 7 - A stated dose per volume
Pool Mate Concentrated Metal Out, 1 quart
Publishes its dose per 10,000 gallons on the listing, which most sequestrants do not, and names the eight metals it is for.
The published figure
Sequestrant dosed at 1 quart per 10,000 gallons; listed for calcium, cobalt, copper, iron, lead, manganese, nickel and silver
Pool Mate states one quart treats 10,000 gallons. That is a real, checkable dose - a 20,000 gallon pool needs two quarts, and you can work that out before you buy rather than after.
A sequestrant does not remove metal from the water. It binds it so it stays dissolved instead of dropping onto plaster as a stain, and it degrades, which is why the label says to re-dose through the season and at closing.
The listed metals include copper and manganese, the two behind most green and purple water that is not algae - see green water that is not algae.
What it does well
- Dose per 10,000 gallons stated on the listing
- Names the specific metals it targets
- Recommended by the maker at opening and closing, which is when it matters
What it costs you
- Will not lift a stain that has already set
- No active ingredient or percentage stated
- A single bottle is one treatment, not a season
Skip it if the stain is already on the wall. A sequestrant prevents; it does not remove. Buy an ascorbic acid product for that.
Pick 8 - Weekly testing you will actually do
AquaChek Select Connect 7-Way Test Strips, complete kit
The kit that gets used. Seven readings in fifteen seconds beats a laboratory-grade kit that stays in the shed.
The published figure
Seven parameters per strip: pH, total and free chlorine, bromine, alkalinity, hardness, cyanuric acid
The best test kit is the one you run. A strip takes fifteen seconds, which is the difference between testing weekly and testing when something looks wrong.
Seven parameters on one strip includes cyanuric acid, which most cheaper strips omit and which is the reading that explains chlorine that will not hold.
Strips are less precise than either Taylor kit and they degrade once the bottle has been opened. Note the date you opened it on the lid.
What it does well
- Fast enough that weekly testing actually happens
- Includes cyanuric acid, unlike most cheap strips
- No reagents to mix or spill
What it costs you
- Less precise than a drop kit, particularly on alkalinity
- Degrades after the bottle is opened, especially in humidity
Skip it if you are diagnosing a problem rather than monitoring. Precision matters when you are about to dose.
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Salt pools close differently
The chemicals are the same - In The Swim's saltwater kit lists components identical to its standard one. What differs is the hardware. The cell comes out, gets inspected and cleaned of scale, and is stored indoors; the controller is set so it cannot fire with no flow. Chemicals do none of that. And because a salt cell pushes pH up all season, the sequestrant matters more on a salt pool than on any other, since high pH is where dissolved metals fall out of solution. The chemistry is on salt vs chlorine.
Above-ground pools close on a shorter list
The chemistry is identical and the kit is smaller - 7,500 or 15,000 gallons. What changes is the hardware. The pump and filter come off, get drained and go indoors, because a soft-sided pool's plumbing cannot be blown out and a frozen pump housing cracks. The water comes down to just below the return, or stays at the skimmer with a winter plug fitted. An air pillow under a solid cover takes up the ice expansion so the walls do not have to.
Because there is no circulation at all once the pump is indoors, the sequestrant and the algaecide are doing all of the work. Dose both at the top of the kit's stated range, brush the walls one last time with a nylon head so nothing is left anchored under the cover, and cover the same day.
Test before you trust the kit
A closing kit is dosed on the assumption that the water is already balanced. It is not a balancer. Test alkalinity, pH, calcium and - on a tablet pool - cyanuric acid a week out, fix whatever is off, and only then add the kit. A strip does that in fifteen seconds and therefore actually gets done, which is why a strip bottle is one of the picks on this page.
The plumbing is the other half of closing, and it has its own page: how to winterize a pool covers blowing the lines, plugs, the skimmer and antifreeze. Leaves are the third half, and they have their own page too: winter leaf control covers the net, the cover, and the spring you get if you skip both.
What a good close looks like in April
Clear water under the cover, a light film on the floor, free chlorine near zero, pH and alkalinity within a step of where you left them, and a first test that calls for a normal dose rather than a rescue. That result costs one afternoon in October and about the price of a closing kit. What you add when the cover comes off is on pool opening chemicals. The alternative - a green pool on opening day - costs a week and several times as much in chemicals, and it is the single commonest way a pool season begins.
Questions people actually ask
What chemicals do I need to close a pool?
What is the best pool closing kit?
What happened to the In The Swim pool closing kit?
Should I shock the pool before closing?
When should I close my pool?
Do I need an algaecide if I use a closing kit?
How much closing kit does my pool need?
Sources
Every figure on this page traces to one of these. Where a number could not be sourced, the page says so rather than estimating.
- Skim & Soak methodology - the constants, formulas and worked examples behind every dose on this site
- HTH Pool Care Shock Advanced - product page, cal hypo formulation, adds no cyanuric acid
- HTH Pool Care Alkalinity Up - product page, states the 80-120 ppm total alkalinity target and a 6-8 hour circulation wait
- HTH Pool Care pH Down - product page, sodium bisulfate, states a 7.2-7.6 target and a 4-6 hour wait after shocking
- Doheny's Master Pool Closing Kit, up to 35,000 gallons, contents itemized - Amazon listing, read 2026-10-03
- Doheny's Ultimate Pool Closing Kit, 15,000 gallons, includes Metal Out - Amazon listing, read 2026-09-12
- Rx Clear Pool Closing Kit, 20,000 gallons, includes Alkaplus buffer - Amazon listing, read 2026-09-12
- Robelle 30,000 Gallon Triple-Action Winter Kit - Amazon listing, read 2026-09-12
- In The Swim Saltwater Pool Closing Kit, 35,000 gallons - Amazon listing, read 2026-09-12
Read next
How to Balance Pool Water
The correction order that stops you doing the same job twice, with computed dose tables and the formulas behind them.
Top pick
Pool Mate Premium Alkalinity Increaser, 10 lb
Raising total alkalinity
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Leaves in the Pool Over Winter
Close clean, cover properly, net the cover, and check it through winter - the four steps that decide your spring.
Top pick
Blue Wave BWC564 Rectangular Leaf Net Cover, 20 ft x 40 ft
Sizing over an existing winter cover
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Best pool cover pump
Best Pool Cover Pump
Cover pumps compared on the specification that defines them - switch-on and switch-off depth - plus the unpowered siphon that is the right answer above ground.
Top pick
WAYNE WAPC250 Automatic Pool Cover Pump
Set it in October, forget it until April
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Best pool metal remover
Best Pool Metal Remover
Sequestrants that hold copper and iron in solution, ascorbic acid lifters that take an existing stain off the surface, and the order to use them in.
Top pick
Pool Mate Concentrated Metal Out, 1 quart
A stated dose per volume
Best for green water
Best Pool Shock for a Green Pool
Cal-hypo, dichlor and liquid chlorine compared on available chlorine percentage, with a dose table computed for six pool sizes.
Top pick
In The Swim Pool Shock, 68% Cal-Hypo Granular, 25 lb
Clearing an algae bloom
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