What Chemicals Should I Put in My Pool When Closing?

As those warmer summer temperatures start to fade away, it’s time to think about closing your pool for the off-season. If the winterising process is on your mind and at the top of your do-to list, one of your main priorities is likely what swimming pool chemicals you need to help get the job done. 

Here, we cover which winter chemicals you’ll require to keep things ticking over during the colder months. 

Preparing Your Pool For The Off-Season

Aside from adding the right chemicals to the water, there are a couple of other key maintenance tasks you’ll need to perform when closing. Following a thorough winterising checklist will help ensure your kit is in good condition when you open back up in spring. 

For guidance on the other essential winterising tasks, please see our full guide to closing your pool for winter. 

A guide to closing your pool for winter: winterising checklist

Test Your Pool Water 

As always, you’ll need to start by assessing the quality and current chemical levels of your water to see exactly what/how much needs to be added. You can use water test strips to quickly and accurately check your chlorine, pH, total alkalinity and calcium hardness levels. 

  • pH levels should be: 7.4-7.6
  • Chlorine levels should be: 3 ppm

What Pool Chemicals Do You Need When Closing For Winter? 

If you’ve kept on top of your regular chemical maintenance, there shouldn’t be too much to do when closing, and you’ll be familiar with the what and the why. The key winter pool chemicals you’ll require include: 

  • Chlorine shock (or non-chlorine alternative if you prefer)
  • Long-life algaecide 

Dark water with leaves floating on it

Chlorine Shock 

One of the main things you’ll do to prepare for winter closure is shocking your pool. A strong dose of chlorine or a non-chlorine oxidiser will kill lingering contaminants, bacteria and algae, so you don’t get anything nasty growing underneath your pool cover only to be revealed when spring rolls around again. 

Removing existing algae and organic pollution gives your algaecide the best chance of success, essentially providing a blank slate for it to keep clean. 

Aim to do this five or so days before fully closing your pool and securing your winter cover, as the shock will need time to circulate and you’ll need to wait until the chlorine levels have dropped to the recommended level before adding any other chemicals. 

Shop for chlorine


Once your water has been treated with a shock (and left for the requisite time), you can add a dose of long-life algaecide. Long-life algaecide products are designed to stay working in your water for up to three months per application. During this time, the algaecide will inhibit the growth of new algae, keeping your water clear and clean. 

You’ll usually come across two kinds of algaecide: copper-based and copper-free. Copper sulphate is very effective at stopping algae from growing in your pool, which is why it’s used. However, adding more copper to pools that already have high metal levels can lead to scaling and staining – if this is a problem for your pool, it’s best to opt for copper-free algaecides, which will still do a great shop at keeping your pool algae-free! 

If you’re using algaecide for winter closure, ensure you make the distinction between long-life algaecide and algae controllers that are designed for more frequent use. One dose of long-life algaecide should take you through a good portion of the off-season, but other algaecides may be intended for more frequent application – you might come back to some nasty surprises if you assume these products are going to keep your pool algae-free for months at a time without reapplication.

Shop algaecide

Winter Pool Chemical Kits 

Winter closing kits can be useful because they provide everything you need for the job, but some kits quickly become unnecessarily complicated, including specialist chemicals you might find you don’t actually need. 

This is why our winter pool closing kit keeps it simple with quality chlorine shock and long-life algaecide, ensuring you’re equipped with the absolute essentials. When you buy this as a kit, the price is less than if you were to buy the chemicals separately. So, if you need to stock up on both for the off-season, we’d recommend getting the two chemicals in the kit!  

A snowflake and frost

Handle Winter Pool Chemicals With Care 

As with your other regular pool maintenance chemicals, carefully read the dosage and safety instructions. Pay attention to the manufacturer's guidance for specific chemicals, as the doses or handling instructions may differ slightly between brands. 

Monitor Your Chemical Levels Over the Winter 

Once you’ve added your winterising shock and algaecide, you can cover your pool with a sturdy winter cover to keep debris and sneaky contaminants out. However, even though it’s covered, it’s a good idea to monitor the water’s chemical levels and test them every now and again to check whether anything needs to be adjusted. 

For a bit of advice on post-winterising tasks like this, check out our top tips for swimming pool maintenance in winter. 

top tips for winter pool maintenance

Ultimately, with the right chemicals and care, you should have no trouble closing your pool this season. For more guidance on winter closure or any other common pool queries, browse our blog.