This blog is about swimming pools, spas and hot tubs and how to maintain them.

Why is there no chlorine in my pool?


A frequent problem that we have customers ring up for is that they can’t detect any chlorine despite adding it to the pool. They add chlorine, test, find none, add more, test…..What is going on? Well, like so many other pool problems, there can be a number of possible causes.

The three most common causes are:

Bleaching of test reagent

In this instance the test reagent being used is bleached out by excessive chlorine levels (normally in excess of 15ppm). If you are using a test strip then the pad will turn white and if you are using a DPD tablet then the sample will remain clear instead of turning pink.

To establish whether this is happening you need to dilute the testing sample. Instead of having your sample made up of 100% pool water, take a sample which consists of 25% pool water and 75% tap water. Carry out the test again and then multiply the result by four to obtain your true level of chlorine in the water.

To reduce the chlorine level you can add Dechlorinator.

Killing algae in the pool.

When a pool goes green the normal method of making the pool water clear and blue again is to add large amounts of chlorine. This is known as shocking the pool. In the process of killing the algae the chlorine is used up and although it appears that you are adding chlorine and it is just disappearing.

Sunny weather and no stabiliser.

Chlorine is broken down by sunlight. This can happen quite quickly even in a British summer making it very difficult to keep free chlorine in the water. The answer is to add a stabiliser or use a sanitiser with a stabiliser included such as Stabilised Chlorine Granules or Stabilised Chlorine Tablets.

Cloudy Swimming Pools


Cloudy water is one of the most common problems pool owners face. There are three main causes of cloudy water.

  1. No chlorine or bromine
  2. Filter not functioning properly
  3. Dissolved air

In this blog post we will advise you on how to restore the sparkling look to your water.

The first thing to do is check your chemical levels and in particular the chlorine, pH, alkalinity and stabiliser (cyanuric acid).

If the chlorine or level is low, shock the pool with 500 grams of chlorine shock per 10,000 gallons of pool water. (For a very cloudy pool it may be necessary to use double this amount or more.) Leave the water to circulate overnight and re-check the chlorine level to ensure it is at least 5 ppm. If the stabiliser level is very high on the test strip then you will need to replace a portion of your water with fresh water to bring it down. For all the other parameters make sure they are within the required limits as stated in the table below.

Pool Spa
Chlorine 1-3ppm 2-4ppm
Bromine 3-6ppm
pH 7.2-7.6 7.2-7.6 (7.2-8.2 if you are using Bromine)
Alkalinity 80-120 80-120

If all your parameters are within range then the cause of the cloudy water is probably down to fine particles of debris or material passing through the filtration system. When the water passes through the filtration system the role of the sand, EGFM or cartridge is to trap any debris or material and prevent it from returning into the pool. Sometimes these particles are so fine that they pass straight through the filter and re-enter the water. These fine particles are the cause of the cloudy water.  This is particularly common when you have had to shock your pool to kill a lot of algae or if you have had a lot of people using your spa.

In order to eliminate these particles and restore water clarity a flocculant needs to be added to the water. The flocculant (also known as a coagulant) enhances the removal of the fine particles by clumping them together so they form a flocculus (floc) which is more easily trapped in the filter.

If after checking your chemical levels and adding a flocculant you still have cloudy water, then you need to check your filtration system is working properly. If you have an above ground pool or spa then remove the filter cartridge and give it a thorough clean. If you have an in-ground pool you will need to check that your pump is working. You can do this by seeing if there is water coming back into the pool via the inlet. If the pump appears to be working then back wash the filter and then remove the lid to check the level of the sand. When checking the level of the sand always use a tape measure as when the filter is full of water it looks as though the sand is closer to the top than it is. Your filter should be two thirds full of sand. Whilst doing this also observe the state of the sand. It should be clean, reasonably level and should have no gaps or holes in it. If the sand level is not as described above, the filter should be emptied and the distributor at the bottom should be checked and repaired if necessary. Finally new sand or EGFM needs to be added to the filter.

The last and least common cause of cloudy water is dissolved air. This is caused by a leak on the suction side of the pump where air is sucked in and dissolved in the water when it passes through the pump. When the water gets back to the pool it forms very small bubbles which are suspended in the water making the pool look cloudy. Pools with dissolved air also tend to foam when the water is disturbed. The most common place for a leak to occur is the lid of the strainer. To check for this turn off the pool pump and close the valves. Remove the strainer lid and examine the seal. Wipe the seal and put some vaseline on the seal before putting the lid back.

The other common place for air to get in is the mechanical seal. Check under the pump for evidence of water leaking. If it is leaking you will either have get a replacement mechanical seal or a new pump.

If the strainer and pump are OK check all the pipework from the pool to the pump for leaks on joints or cracks in pipes.

Why is shocking your pool important?

Shocking your pool is one of the most important procedures in swimming pool maintenance but we often get a lot of customers asking why it’s so important? So in this blog post I will explain.

The main reason why you need to shock treat your pool is to prevent algae growing in the water. Algae are microscopic plant life that grows very quickly and rapidly in sunny and warm conditions and if they are not killed you will soon be faced with a horrible green mess instead of a nice sparklingly clear pool! The truth is nearly all outdoor pool owners in the UK will have encountered this problem in the past. No matter how good your filtration system is or how vigilant you are with your chlorine levels algae spores will still find a way into your pool water. Commonly this is via dirt from rain or bathers.

Maintaining a free chlorine level between 1 and 3 parts per million (ppm) will kill the majority of algae spores that enter the water but over time the remaining algae spores will build up and develop into algae blooms (green mess). This is especially true in sunny and warm conditions as mentioned earlier. This is where shocking your pool becomes imperative. Once a fortnight, or once a week in the summer, you should use Calcium Hypochlorite Granules (Chlorine Shock) to super-chlorinate the pool and increase the chlorine level up towards 10ppm. This rapid increase in chlorine will obliterate any algae spores that are in the water and it will ensure you maintain a sparklingly clear pool. Remember to let your chlorine level drop back into the recommended range before re-entering the pool.

Just a quick note on why you need to use Calcium Hypochlorite Granules (Chlorine Shock). Calcium Hypochlorite is an unstabilised form of granular chlorine that is most commonly used as a shock treatment in swimming pools. It has 70% available chlorine which makes it stronger than other chlorine based chemicals such as Sodium Hypochlorite (14-15%) and Stabilised Chlorine Granules (56%). Another major advantage of using calcium hypochlorite as a shock treatment is that it doesn’t contain any stabiliser. There can be occasions where you need to add a large shock dose in order to kill algae. If you used stabilised chlorine like the aforementioned Stabilised Chlorine Granules, then it would still kill the algae but it would also increase the stabiliser level in the water. Over time the stabiliser level will build up and eventually it will get too high and you will have to reduce it by draining a proportion of pool water and replacing with fresh water.

Another reason why shocking your pool is important is to reduce the combined chlorine level. This is more applicable to public pools that experience a high bather load but it is important for domestic pool owners to be aware of it as well. When you add chlorine to the pool water it produces hypochlorous acid (HOCl), which is commonly referred to as free chlorine. As noted earlier the free chlorine is responsible for killing bacteria, viruses and algae spores in the pool water but even if you maintain a suitable free chlorine level (1-3ppm) it is inevitable that some pollution will still enter the water. This could be in the form of dirt and algae spores from the rain or perspiration, oils and cosmetics from bathers.

When the pollution enters the water a chemical reaction will take place with the free chlorine. This reaction produces combined chlorine which consists of the chloramines called mono-chloramine, dichloramine and nitrogen trichloride. The mono-chloramine is responsible for the distinct chlorine swimming pool smell and the nitrogen trichloride is very irritating to bathers eyes and noses. Although combined chlorine contains a lot of chlorine (hence the smell) it is all locked away therefore it does not kill bacteria, viruses and algae. So in summary you do not want combined chlorine in your pool.

There are ways in which you could minimise the combined chlorine level without using any chemicals. For example you could use a pool cover when the pool isn’t in use and you could insist that bathers showered before using the pool. Sounds straightforward but the first proposal can be expensive and we all know the latter just won’t happen. Therefore it is important to reduce the combined chlorine level by regularly shocking your pool with an oxidising agent such as Calcium Hypochlorite (Chlorine Shock) or Oxy Shock (Potassium Mono-Peroxysulphate). The addition of either of these chemicals will remove all pollution and reduce the combined chlorine level. In normal circumstances once a fortnight should be adequate but if you experience a high bather load, sustained period of hot weather or a substantial rainstorm then you may need to shock your pool once a week.

Finally although I have talked about combined chlorine you don’t need to worry too much about testing for it. Providing you shock your pool regularly a domestic family pool should not encounter any of the problems caused by combined chlorine. However if you are interested in testing for combined chlorine you can revisit a previous blog.

Thinking of opening your pool up?


As we move into March most pool owners across the country will start thinking about pulling back the pool cover and getting the pool ready for the spring. If you winterised the pool at the beginning of last winter then opening the pool should be a relatively straightforward process, but nevertheless here is a simple step-by-step guide on how to get your pool ready.

Preparing the Pool

  1. Remove any leaves and debris from the winter cover. Take the winter cover off, clean it and store it away.
  2. Connect the pool pump and filter and reconnect all hoses and electrical connections.
  3. Remove leaves and debris from skimmers, filters, pumps and drains.
  4. Remove any debris and leaves that are in the pool using a leaf skimmer.
  5. Fill the water up to the proper water level. The water should be halfway up the skimmer opening.
  6. Turn on the filter pump and make sure all skimmers, bottom drains and filters are functioning properly.

Adjusting the pH and Alkalinity Levels

  1. After the pool filter has been running for 3-4 hours, test the pH and alkalinity levels and compare with the table below.
  2. To reduce the pH level use pH Reducer. Dissolve a small amount (450g per 10,000 gallons) in a bucket of pool water and distribute it around the pool with the filter operating. Retest after 4 hours.
  3. To increase the pH level first adjust the Alkalinity to between 60-100ppm. To increase the Alkalinity level by 12ppm add 1kg Alkalinity Builder per 10,000 gallons directly to the pool with the filter operating. Retest the Alkalinity level and the pH level after 4 hours.
  4. If the pH level is still too low dissolve a small amount (450g per 10,000 gallons) of pH Increaser in a bucket of pool water and distribute it around the pool with the filter operating. Retest after 4 hours. Repeat this dosage if necessary.
Low OK Ideal OK High
pH Less than 7.0 7.0-7.2 7.2-7.6 7.6-7.8 Over 7.8
Total Alkalinity Less than 40ppm 40-60ppm 60-100ppm 100-120ppm Over 120ppm

Shock Treat the Pool

  1. Brush the pool walls and steps and vacuum up any debris. This will expose the algae to the chlorine.
  2. Dissolve 500 grams per 10,000 gallons of Chlorine Shock in a bucket of pool water and distribute around the pool. DO NOT ADD TO THE SKIMMER BASKET IF IT HAS HAD STABILISED CHLORINE IN IT AS IT MAY EXPLODE IF MIXED!!!
  3. Depending on how green the pool is it might require several shock treatments. If your pool is very green you can double the initial shock dose. Following the shock dose, backwash the filter to remove any debris.

Stabilising the Chlorine Level

  1. After a few days your chlorine level should have returned to normal (see table). Once it has you can now add a 200g Multifunctional tablet or 200g Stabilised Chlorine tablet to your skimmer basket or dispenser.
  2. To improve the clarity of the water add Sparkle Water Clarifier to a bucket of pool water and distribute around the pool.
  3. To prevent algae re-occurring add Polyquat Algaecide to a bucket of pool water and distribute around the pool.
Low OK Ideal OK High
Chlorine Less than 0.5ppm 0.5-1.0ppm 1.0-3.0ppm 3.0-5.0ppm Over 5.0ppm

Daily Maintenance

  • Monitor Chlorine, pH and Alkalinity levels using AquaChek Free Chlorine Test Strips.

Weekly Maintenance

  • Clean the skimmer baskets and brush the walls, steps and bottom of the pool. Vacuum if necessary.
  • Clean the strainer basket before the pump. When this is complete backwash the filter.
  • Shock dose the pool. Dissolve the required amount (see instructions) of Chlorine Shock in a bucket of pool water and distribute around the pool. DO NOT ADD TO THE SKIMMER BASKET IF IT HAS HAD STABILISED CHLORINE IN IT AS IT MAY EXPLODE IF MIXED!!!
  • Add Sparkle Water Clarifier to a bucket of pool water and distribute around the pool.
  • Add Polyquat Algaecide to a bucket of pool water and distribute around the pool.

Dosing Information – Health and Safety

  • Always wash out the container you mix chemicals in thoroughly with pool water before and after use.
  • Do not ever mix chemicals together
  • Always add chemicals to water not water to chemicals.

Step-by-Step Guide to Replacing your Hot Tub Filter Cartridge


Trying to find a replacement filter cartridge for your spa or hot tub can be an arduous task due to the number of filters currently on the market. Not only are there different filters for different manufacturers but there are also different filters for different models by the same manufacturer! We hope that this blog post will help you find the filter that you are looking for.

The first thing you need to do when finding a replacement hot tub filter is to have your old filter to hand. Once you have it there are several ways in which you can identify the filter you need.

Reference Number

When searching for a replacement filter cartridge you will normally come across four main manufacturers which are Darlly, Filbur, Pleatco and Unicel. If you look on the top of your old filter most manufacturers of filter cartridges will state the manufacturers name and the filter code. These two bits of information can then be used to find your filter.


If you cannot find a reference number then you can use the measurements of your filter cartridge to find a replacement. When measuring your filter always take measurements of the length, diameter and any holes that appear on the top or bottom. It is also important to take note of any connections on the bottom of the filter as the replacement you buy will need to have the same connections.

Measure the length of the filter

Measure the diameter of the filter

Measure the diameter of any holes at the bottom

Measure the diameter of any holes on the top





Hot Tub Manufacturer

Finally if the two previous methods don’t work you can also find your replacement filter by searching for filters suitable for your hot tub manufacturer. However as mentioned earlier in some instances there are several filters available for the same manufacturer so try and have either a reference number or measurements available as well.

We have a wide range of replacement filter cartridges available to buy on our website

Algae in the corners of a tiled pool


If you have algae growing in the corners of your tiled pool it can often be difficult to remove by just doing a normal shock dose. This is because the circulation of the water often doesn’t reach those areas and hence chlorine doesn’t either. Any chlorine that you add will kill any micro organisms and algae in the main body of the pool but it may not kill any algae in difficult to reach areas such as the corners of the pool. If left untreated the algae can multiply rapidly and you could soon be faced with a pool that is completely green.

To kill any algae in the corners of your pool it is recommended that you shock the pool with calcium hypochlorite (500 grams per 10,000 gallons) as normal. However in addition to this put some gloves on and sprinkle some calcium hypochlorite granules directly into the corners where the algae is situated. Using an algae brush, brush the granules over the algae. The combination of physical cleaning and the very high local concentration of chlorine should kill the algae and remove it from the corners. When you have got rid of the algae add 1 litre of polyquat algaecide and then 100 millilitres per week to prevent recurrence.

Foaming in Spas and Hot Tubs


One of the most common problems spa and hot tub owners get is a layer of foam on the surface of the water. This is caused by a reaction that takes place between the natural alkalinity of the water and the bathers body oils. If the bather hasn’t showered before getting in the water then any cosmetic products, suntan lotions or detergents will wash off into the water. Obviously this can be prevented by showering before getting into the water, however this is not always possible. With this in mind you may need to add Antifoam to get rid of any excess detergents. Antifoam is an inert silicone based product used to effectively control foaming in hot tubs and spas. Adding approximately 50ml directly to the spa or hot tub should solve the problem.

Another cause of foaming is soft water. If you have a low calcium hardness level then this needs to be increased to at least 200 ppm to prevent foaming.

Maintaining a Hot Tub in a Hard Water Area


If you take a look inside your kettle do you see a build up of scale? If you do then the chances are you live in a hard water area. If you have a hot tub and you live in a hard water area then there are various things that you need to be aware of that will affect the running of your hot tub.

Hard water occurs due to the rain water filtering through soluble rock such as limestone and chalk, resulting in mineral salts like calcium dissolving into it. In terms of your hot tub water this usually means that you will have a high calcium level, high pH level and high alkalinity level. High levels of calcium are not good for your spa. Not only can calcium deposits (scale) appear on the surface of your spa, but scale can also build up on internal pipe work and heater, just like it does on the inside of your kettle. This can lead to substantial damage if not treated accordingly with a scale inhibitor.

The main chemical that makes water hard is calcium bicarbonate. The calcium part is the hardness whilst the bicarbonate is the alkalinity. As a result you will probably also have a high pH level and high alkalinity level. Both of these issues can be rectified by adding a pH Reducer, which is usually sodium bisulphate.

There are a couple of additional things you can do to help if you do live in a hard water area. From a chemical perspective it may be prudent to run your hot tub on bromine as you can maintain a pH level between 7.2 and 8.2. If you run on chlorine then you must keep the pH level between 7.2-7.6 so it is more difficult to sustain a suitable pH level.

Finally you can try and remove calcium from your water supply when you refill your hot tub by using the Darlly Pure-Stream Pre-Filter. This is an ion exchange resin pre-filter which attaches to your hose pipe and removes unwanted calcium and magnesium from your incoming water supply and replaces it with soft sodium instead.

Sea lion takes a dip in public swimming pool


This article caught our eye in the Daily Mail on Monday. Swimmers at an open-air public swimming pool in Dunedin, New Zealand were given a surprise recently when an unexpected guest turned up. At around 2.30pm holidaymakers and locals were joined in the pool by a sea lion who had casually made its way past reception and cafe before taking the plunge into the saltwater pool. Although New Zealand sea lions are known not to fear humans, swimmers quickly evacuated the pool and after a nice leisurely swim, the sea lion made its way out of the pool and back to its normal surroundings.

Hot Tub Starter Kits


Happy New Year to you all!

If you are thinking of purchasing a hot tub, shortly after installing it you will need to purchase some chemicals in order to keep the water safe to bathe in. If you are not experienced in maintaining a hot tub it can be daunting when faced with all the different chemicals. What are they? What do they do? How much do I need? These are probably just a few of the questions running through your mind. However, there is a simple answer to all these questions. Buy a chemical starter kit!!

Starter kits offer a cost effective and hassle-free method for ensuring you get your hot tub up and running and safe to bathe in. The basic starter kits on the market will contain chlorine (or bromine), a shock treatment, water balance chemicals and some test strips. These are the essential chemicals that you need to get your hot tub started. However, it is important to note that when maintaining a hot tub the likelihood is that you will need to use additional chemicals in the future, such as a water clarifier, filter cleaner, antifoam and perhaps a scale inhibitor if you live in a hard water area. It is therefore advisable to purchase a starter kit that also contains these additional chemicals. It may be more expensive at the start but you will save money in the future. Also if you do encounter any problems, such as foaming, you will have the peace of mind of knowing that a short trip to the garage or shed will sort the problem out quickly.

All good starter kits should also come with an instruction leaflet on how to run a hot tub. These are often quite extensive and provide a great deal of valuable information. This will help to ensure that maintaining your hot tub is a hassle-free experience.

We have a range of hot tub starter kits on our website, all of which include clear instructions on starting and maintaining your hot tub. Please click here for more information. If you are unsure on whether to use chlorine or bromine then please read our relevant blog post on this subject.