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

Calcium Hypochlorite Granules (Chlorine Shock)


Calcium Hypochlorite (Chlorine Shock) 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 shock treatment 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 a 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.

Calcium hypochlorite granules should be dissolved in a bucket of warm water and distributed around the pool or poured into the skimmer basket. HOWEVER please make sure your skimmer basket is free from all other types of chlorine before pouring it in. The reason for this is if you mix unstabilised chlorine and stabilised chlorine together in their raw form they will react violently. Once they are added to the pool separately they are fine. People often use a trichlor tablet as their sanitiser and calcium hypochlorite granules as their shock treatment. If this is the case put the trichlor tablet in the skimmer basket and when it is time for a shock treatment, dissolve the calcium hypochlorite granules in a bucket of warm water and pour the solution around the pool. This will ensure you keep the two types of chlorine separate from one another.

It is not recommended to use calcium hypochlorite granules in a spa as it is undesirable to have large amounts of calcium build up in a spa. It will cause scaling of the heater and white deposits on the sides therefore we recommend using Oxy Shock instead.

How does your water supply affect your Hot Tub maintenance?


Did you know that your water supply plays a significant role in how you go about maintaining your hot tub water? The water supply in our homes is usually referred to as hard or soft and it is important to know your water supply as it will affect some aspects of your hot tub maintenance.

A quick way to determine whether you have hard or soft water is to take a quick look in your kettle. If you can see signs of scale then your water supply will be classed as hard. If you can’t then it is likely to be soft. If you are unsure you are best to check with your local water board.

If you live in a hard water area then you need to be aware of two main issues; scale and a high pH level. I mentioned earlier about the build up of scale in your kettle. The chances are it will be a similar picture on the internal pipe work and heater in your hot tub. If left untreated then this could eventually cause significant damage, especially to the heater. With this in mind it is important that you use a scale inhibitor such as our Stain and 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 the second problem you will probably encounter is 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. If you do live in a hard water area 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 maintain a pH level between 7.2-7.6.

If you live in a soft water area then the opposite will happen as your water supply will have a low pH level and low alkalinity level. First of all you need to get your alkalinity level right and you can do this by adding an alkalinity builder, which is sodium bicarbonate. Once you have added this you will find that it slowly also brings your pH level into the desired range.

Trick or Treat?


With Halloween fast approaching we thought we would share some pool and spa tricks with you that will hopefully help you when maintaining your pool or spa.

  1. If you have an indoor pool, consider using unstabilised chlorine instead of stabilised chlorine. This will prevent the stabiliser level from getting too high and prevent the need to dump any water.
  2. When dosing granular chlorine always dissolve the required amount of chlorine into a bucket of water before adding it to the pool. This will ensure that no chlorine goes to waste and also prevent any bleaching of the pool liner.
  3. When altering the pH level always ensure the alkalinity level is correct first as this has an effect on the pH.
  4. If you have a green pool you may need to double or even treble the normal shock dose in order to kill the algae. As there is no way of measuring algae it can be difficult to give an exact amount but always overdose rather than under dose or else you could end up going round in circles!
  5. When buying Chlorine Shock just purchase enough for one season. Due to the nature of the chemical if it is kept in storage for too long it can start to attack the packaging, making it brittle and weak.
  6. Over the winter in addition to winterising your pool, keep a large floating dispenser in your pool with a 200g Multifunctional Tablet in it. This will further enhance the condition of your pool when you open it up again in the spring.
  7. After refilling your spa or hot tub, use either Bromine Spa Starter or Chlorine Granules to quickly establish a bromine/chlorine level. If you only use tablets it will take a while for you to establish a suitable sanitiser level in your spa.
  8. Use Oxy Shock to shock treat your spa as it is faster dissolving than chlorine shock and it fully dissolves. Also it doesn’t produce the chlorine smells you get with a chlorine based shock.
  9. To improve the clarity of your water, shower before using the spa. This will get rid of any lotions and perspiration that would normally cloud the water.
  10. Drain and refill your spa on a regular basis. As a rough guide a 1000 litre spa should be drained after 100 bathers.


Cow rescued from swimming pool

We recently came across this interesting article on the BBC.

A cow had to be rescued by firefighters from a swimming pool in Cheshire after falling through a pool cover after it mistook it for a solid surface. Firefighters had to pump out water from the pool before using bales of hay to create steps, which allowed the cow to get out of the pool and return to the safety of dry land.

Winterising your pool


With the school holidays over and autumn on the horizon, many pool owners will start thinking about closing their swimming pool down for the winter. This procedure is very important for two reasons. Firstly it helps ensure your pool is in the best condition possible when you come to open it up again in the spring and secondly it helps protect your pool and its fittings from the winter conditions. You can close your pool down by following the simple steps below.

  1. Backwash the filter and clean the strainer.
  2. Check the pH is between 7.2 and 7.6 and use sodium bisulphate (to lower pH) and sodium carbonate (to increase the pH) to adjust if necessary.
  3. Using Chlorine Shock (calcium hypochlorite), boost the chlorine level to remove any algae that maybe present in the pool. The recommended dosing rate is 1kg per 10,000 gallons of pool water.
  4. To prevent algae from growing add Winterising Algaecide. The recommended dosing rate is 2.5 litres per 10,000 gallons of pool water.
  5. Turn on the circulation to disperse the solution through the water for a period of 24 hours.
  6. Reduce the water level to just below the skimmers. This is to allow for any extra water that enters the pool, ie rainfall, therefore preventing it from overflowing. WARNING: Do not lower the level too much especially if the water table is high as this could cause structural damage to the pool shell.
  7. To protect any exposed metal surfaces, drain the water from the pump(s), filter(s), heater and pipework.
  8. Leave any valves above the water level open. Any below should be closed to protect from frost damage.
  9. Switch off the electricity supply to the system.
  10. Cover the pool with a heavy-duty winter pool cover.
  11. Check the appearance of the water, and the chlorine/pH levels occasionally during the winter.

By following this set of guidelines it should significantly reduce the amount of work you have to do when opening the pool up in the spring. Winterising Kits are available from our website which include all the chemicals you need as well as step-by-step instructions.

Please note the information provided on this page is believed to be correct but is given without warranty and no liability is accepted or implied. Please use this information in conjunction with manufacturers instructions.

What is calcium hardness?

When dealing with pool water the term ‘hardness’ can be somewhat confusing! Obviously water cannot be hard in a conventional sense unless it is frozen, so what does it mean and how does it affect the running of a swimming pool? Put simply hardness is the concentration of calcium and magnesium salts in the water. For the purpose of swimming pools we shall only deal with calcium in this post.

A rough guide to water supplies in the UK.

The water supply to our homes is often described as hard or soft.

Hard water

Hard water occurs due to the water supply filtering through soluble rock such as limestone and chalk, resulting in mineral salts like calcium dissolving into it. Water supplies in the south east of England are normally hard and this can lead to scale appearing on kettles and pool equipment. Hard water also stops your soap foaming as well. The main chemical which makes water hard is calcium bicarbonate. The calcium part is the hardness whilst the bicarbonate is the alkalinity. As a result hard water has a very high pH level.

Soft water

Soft water has very little minerals in it and as a result is very hungry to dissolve metals such as lead as well as concrete or grout in pools. To stop this from happening you will need to add calcium to the water which is discussed in more detail later on in this post.

Calcium hardness

Calcium hardness is a measure of the amount of calcium dissolved in water.

Why is calcium hardness important in swimming pools?

Most swimming pools are made of concrete and are often tiled with grout between the tiles. Pool equipment, particularly pumps and heaters are made of metal. As mentioned earlier soft water is hungry to dissolve anything soluble it comes into contact with so it will dissolve the concrete, grout and metals until it is satisfied. Therefore it is necessary to add calcium to the water to prevent this from happening.

What should the calcium hardness level be?

The ideal water for swimming pools is medium to hard water. The PWTAG publication “Swimming Pool Water – Treatment and Quality Standards for Pools and Spas” recommends the following concentrations of calcium in pool water:

  • Minimum 80 ppm
  • Maximum 150 ppm

Pools in hard water areas have over 200 ppm of calcium in the water supply so they will never have to add calcium. Pools that chlorinate using calcium hypochlorite will usually operate with calcium concentrations well over 150 ppm. This will not do any harm in a swimming pool unless it is very high (over 500 ppm).

PLEASE NOTE – These levels apply to swimming pools they do not apply to domestic hot tubs. Water hardness in hot tubs is discussed in a separate blog post.

The loss of grout on swimming pools is often blamed on low concentrations of calcium hardness in the water. Pool builders often recommend boosting the calcium hardness level to as high as 400 ppm to prevent grout loss. PWTAG carried out research on this and could find no evidence to support using a higher concentration of calcium hardness.

How to adjust calcium hardness

To increase the calcium level you need to add calcium builder (calcium chloride) or use calcium hypochlorite as your source of chlorine.

1.5kg of calcium chloride will increase the calcium hardness level by approximately 20ppm in a 10,000 gallon (45m3) pool.

In a soft water area the calcium level can be decreased by diluting with mains water (backwashing). In hard water areas the only way of decreasing the calcium level is to use a water softener which is not recommended or necessary in swimming pools.

How to test for calcium hardness

Calcium hardness can be tested using test strips, a comparator, a photometer or the traditional tablet counting method, which in my opinion is more accurate than either of the aforementioned methods.

Please note once you have established that your water supply is hard, you rarely need to check the calcium hardness level.

Tablet Count Testing Method:

  1. Fill a dilution tube with 100ml of pool water.
  2. Add calcium hardness tablets one at a time and shake the sample to ensure they are dissolved.
  3. When the water changes colour from pink to bright purple the test is complete.
  4. The calcium level is obtained by the following formula; (the number of tablets used  x 20 ) – 10

For example if 7 tablets are used the calcium level is (7 x 20) – 10 =  130 ppm


Looking after your pool in the rain


With all the rain around recently the last thing you have probably felt like doing is going outside for a swim and who could blame you! However it is still important that you look after your pool so when the sun reappears it will be in top condition and ready for you to use.

The reason for this is because during spells of heavy rain the rain washes dust and algae spores into the pool and uses up the chlorine. When the chlorine disappears algae will start to thrive and before you know it you are faced with a horrible green swamp instead of a nice sparkling pool. So instead of enjoying a nice relaxing swim you are having to shock treat it with a lot of chlorine to try and get rid of all the algae.

It is therefore important to take pre-emptive action to prevent this by following these simple steps;

  • Give the pool a shock dose of 500grams  of calcium hypochlorite granules (shock chlorine) for every 10,000 gallons of pool water. You can use stabilised chlorine granules if you have an above ground pool.
  • Add an algaecide to your pool.
  • If you use stabilised chlorine tablets or multifunctional tablets put an extra tablet in the skimmer and run the pool at the high end of the chlorine range.

Even though you are unlikely to use your pool during bad weather, using some extra chemicals now will save you a lot of effort and chemicals in the medium term and it will allow you to use your pool when the good weather returns.

What is Stabiliser?


If you look at your test strips you may notice a pad that tests for stabiliser, but what is it and how does it effect the way you maintain your pool?

A stabiliser is added to the pool water to reduce chlorine loss due to ultraviolet light from the sun. It is usually added to the water via a chemical called cyanuric acid. “According to research results, pools without stabiliser lose about 90% of their total chlorine residual on a sunny day in two or three hours. Pools with 25-50ppm of cyanuric acid, however, under the same conditions, lose only 10 to 15 percent of their total chlorine.” (White et. al. , 1972)

You may not have heard of or even used cyanuric acid before as it is already included in the majority of chlorine products on the market. You may be familiar with chlorine based products such as Stabilised Chlorine Granules, Stabilised Chlorine Tablets or Multifunctional Tablets (a form of the stabilised chlorine tablet). All these products contain cyanuric acid which is why they are referred to as stabilised chlorine. Therefore as well as dosing chlorine into the water they also add cyanuric acid at the same time.

As mentioned earlier only a small amount of stabiliser (25ppm) is needed and the stabiliser level is acceptable up to a level of 100ppm. Once the level exceeds this amount the chlorine becomes less effective. If this happens then you have two choices. You can either switch to an unstabilised form of chlorine such as Calcium Hypochlorite or Sodium Hypochlorite until the stabiliser level drops, or you can replace a proportion of your water. If you choose the latter option then drain your pool by two thirds and replace with fresh water immediately. Do not wait to refill the pool as this could cause damage to the structure of your pool. Always check with your pool builder before emptying your pool.

Indoor pools

If you have an indoor pool that is not exposed too much sunlight, you should consider using an unstabilised form of chlorine such as Calcium Hypochlorite Tablets or Sodium Hypochlorite. There is no need to add cyanuric acid to an indoor pool and if you do the stabiliser level will build and eventually you will have to drain some of your water.

Pool Blaster Catfish Pool Cleaner


SP993MIf you are looking for a pool cleaner that is quick and convenient to use then look no further than the Pool Blaster Catfish from Water Tech.

The Catfish is totally independent from the main pool filtration system which means there is no need for any hoses, cables or additional pumps. It works off a rechargeable Nickel Metal Hydride battery which has a run time of 45 minutes when fully charged. It attaches to all standard telescopic poles making it suitable for all types of pools and spas. It has a 8″ vacuum head and a reusable filter bag which collects leaves, algae and even fine particles such as sand.

It is available for just £139.99 from our website.

Above Ground Pool Chemicals – Water Testing


This week we have taken you through all the major chemical parameters that you need to control on an above ground pool. Now we are going to take you through testing these parameters. Testing your pool water on a daily basis is crucial to provide a clean, safe and comfortable bathing environment for all bathers.

When testing the pool water you are testing for the following:


Reason for Testing: To ensure there is enough chorine in your pool to provide a safe swimming environment.
Recommended Level: 1-3ppm
Level too Low: Open the collar more on your floating dispenser or add stabilised chlorine granules
Level too High: Remove the floating dispenser from the pool and do not add any more chlorine. Remove the pool cover and chlorine will come down on its own.


Reason for Testing: To allow the chlorine to work to its full potential and provide comfortable bathing conditions for the bather.
Recommended Level: 7.2-7.6
Level too Low: Add pH Increaser
Level too High: Add pH Reducer

Other test parameters include Alkalinity and Stabiliser (Cyanuric Acid). The Alkalinity level is not very important on above ground pools but if you are experiencing low levels of alkalinity add a small amount of Alkalinity Builder. If your alkalinity level is too high then it can be reduced using the pH Reducer. As above ground pools are only up for the summer the stabiliser level will not reach a level that will cause concern. In the unlikely event that it does all you need to do is replace a portion of the pool water with fresh tap water.

How do I test for these?

When testing, it is important to have a testing method that is both accurate and reliable. For above ground pools a straightforward testing method such as test strips or a pool tester kit is more than sufficient.

Test Strips

730aTest strips are a popular method of testing the water in above ground pools due to the fact that they are quick and easy way to obtain accurate results. AquaChek test strips are renowned as one of the worldwide leaders in the test strip industry.

The general testing procedure is:

  • Dip your strip into your pool water and remove immediately.
  • Wait 15 seconds.
  • Compare the colour pads to the colour chart on the bottle.

Testing Tips

  • To get the best out of your test strips, store them in a low humidity environment at room temperature.
  • Keep wet fingers out of the bottle to enable accurate results when testing.
  • Do not use any test strips that are out of date.
  • Keep a record of your results, as they can be an excellent reference point when talking to dealers or professionals. They also help you to understand what is going on in your pool or spa.

Pool Tester Kits

An alternative way to test your pool is with a test kit. A test kit normally consists of a clear container split into two sections, one for chlorine and one for 714pH. Each section is filled with pool water and then a DPD 1 tablet is added to the chlorine section and a Phenol Red Tablet is added to the pH section. Once added the water will change colour and this is then compared to the colour chart on the front of the container to obtain your chlorine or pH level.

This week we have taken you through all the important points that you need to be aware of when maintaining an above ground pool. There are other chemicals that you may come across such as algaecide and flocculant and if you would like to find more out about these chemicals you can visit our guide for above ground pools.