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

Flooding of Outdoor Pools


Now the weather is improving a little, thoughts are turning to the opening and re-commissioning of outdoor pools, both domestic and the few public ones. Everywhere the water table will be high and some liner pools will be floating. Pools cannot therefore be drained and so chemical cleansing will have to be done, more than ever.

One thing that causes concern is the flooded pools, many of which are already green with algae because of the mild winter and may well have all sorts of sewage and field run off in them. It would be a wise precaution to carry out super-chlorination of all pools in advance of their use by bathers so as to try and inactivate any cryptosporidium that may be present.

It would be appropriate to dose at 20 mg/l for at least 13 hours with the pump and filter running.  The quantity of chlorine required will depend on how much dirt is in the water. The quantity of shock chlorine (calcium hypochlorite) required is likely to be at least 3kg per 10,000 gals or if using sodium hypochlorite 15 litre per 10,000 gals. There should be no problem in using these quantities of chemicals in a tiled pool, but if it is a liner pool, check with the liner manufacturer and take particular care with older liner pools.  Please note that in theory, it is possible to reduce the concentration to say 10 mg/l and double the contact time.  However, it is best to use maximum chlorine concentration to inactivate cryptosporidium.

It is important to check the free chlorine residual (DPD1) every few hours (using dilution method to avoid any bleaching out). It may be necessary to add more chlorine for very dirty water. Also since both forms of hypochlorite will tend to increase the pH of the water, it is essential to keep the pH down towards 7.2 in order to maintain chlorine activity (ie hypochlorous acid).

Bathers should not enter the pool until the chlorine level has fallen to 5 ppm and it is best to do that by natural decay by leaving any covers off the pool rather than trying to use sodium thiosulphate and running the risk of removing all the chlorine, or indeed creating a chlorine demand.


What is a comparator test?


Comparator Test Kit with Disc

If you are looking for an alternative to test strips when it comes to analysing your pool or spa water then you might consider using a comparator test kit. A comparator is an item of testing equipment used to visually test pool water for parameters such as free chlorine, total chlorine, total bromine and pH. Usually they come in the form of a comparator disc and unit but there are more basic kits on the market that don’t have a disc and just have a colour scale on the side of the test tube.

To conduct a comparator test the first thing you need to do is rinse the test tubes out with tap water. This removes any traces of previous tests that could affect the result. You then need to take two 10ml samples of water. One will be used as a blank in the comparator unit and the other will be used to perform the test. You then need to add the reagent to the sample and crush it using a crushing rod. When doing this you will see that the sample changes colour (if you are doing a chlorine test then it will hopefully turn a shade of pink). Once the tablet has fully dissolved you need to hold the unit up to the light and look through the view finder on the comparator unit and turn the comparator disc until the two colours match. When you have achieved the closest match, read the relevant number off the disc to determine the level in the water.


Basic Comparator Test Kit

There are normally three tests you can do using a comparator and these are;

  • Free Chlorine or Bromine using a DPD 1 tablet.
  • Total Chlorine using a DPD 3 tablet.
  • pH using a phenol red tablet.

If you are using a basic comparator test kit then the process is similar but you don’t use a blank sample. Instead you fill the relevant compartment up with water, add the reagent and then compare the colour of the sample to the colour chart on the face of the compartment.


Bromine is more effective at a broader pH range in spas and hot tubs


Maintaining the correct pH level in a spa or hot tub is crucial. One of the reasons why it is so vital is to ensure that the sanitiser works to its full potential, which in turn makes sure your water is safe to bathe in. One of the big advantages of using bromine as your sanitiser instead of chlorine is you can maintain a pH level between 7.2 – 8.2  instead of 7.2 – 7.6. The reason for this is that bromine is an active sanitiser at a higher pH level so it will continue to kill any bacteria in the water even when the pH level goes beyond 7.6. On the other hand chlorine loses its killing power when the pH level goes over 7.6. Therefore using bromine allows you more flexibility when controlling the pH level which is particularly beneficial for hard water areas where the incoming water supply naturally contains high pH and alkalinity levels.

The environmentally friendly pool filter media


If you are refurbishing your pool filter over the next few months are you aware that there is now an alternative filter media to silica sand, which is becoming increasingly more popular amongst pool owners?

Eco Glass Filter Media (EGFM) is the environmentally friendly and cost effective filter media that is made from recycled glass such as old wine bottles and jam jars. Not only is it environmentally friendly but it also offers many other advantages over silica sand.

EGFM is more efficient allowing you to save on backwashing and the resultant water, energy and chemical treatment costs.

  • Its angular to sub-angular particle shape and bound silica content provides a 30% improvement in turbidity removal over silica sand.
  • It generally removes finer particles from water than the equivalent grade of silica sand.
  • Its non porous composition means it is less likely to ‘clump’ or channel than silica sand.
  • With superior permeability, back washes take less time, saving water/sewer charges, and energy and chemical treatment costs.

EGFM is cleaner as it is less susceptible to bio-fouling. Unlike silica sand EGFM particles have a smoother surface, so bacteria cannot get trapped in any cracks or flaws, which means less remedial action and again less chemical treatment to destroy pollutants.

EGFM requires less material as it is less dense than sand, requiring 15% less media to fill the equivalent filter.

EGFM doesn’t degrade therefore has a longer life span and may not need changing even when the filters are due for refurbishment.

As you can probably imagine there is a sophisticated production process in place which turns your old wine bottles into a safe and effective filter media. EGFM is manufactured by a mechanically induced high-speed process that transfers energy into the glass feedstock resulting in the destruction of glass whilst rendering it sharp free. It does not grind, mill, hammer or flail the glass. It does not abrade, so EGFM retains a smooth shiny surface. The end result is three grades of EGFM which are illustrated below. If you are interested in purchasing any EGFM please visit our website.

Grade 1   Grade 1 (0.5mm – 1.0mm (16/30)). This is the equivalent to 16/30 filtration sand.
Grade 2   Grade 2 (1.0mm – 3.0mm). This acts as support media.
Grade 3   Grade 3 (3.0mm – 7.0mm (pea gravel)). This also acts as support media.

The swimming pool with 660,500 gallons of water!


We recently came across this article on the Daily Mail website from 2012 which features a recreational pool called Nemo 33 in Brussels, Belgium. The reason why it grabbed our attention is because it has a deep end measuring an unbelievable 34.5 metres and it holds a staggering 660,500 gallons of water!! A bit different to the 10,000 gallon pools we deal with on a daily basis!! The reason for the 34.5 metre drop is because the facility is also used a training facility for divers as well as a recreational pool. Therefore diving is allowed in this swimming pool!

Closing your pool for winter


It is around this time of the year when pool owners 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.

Why do you need to shock your pool?


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.

How does your water supply affect your Spa or Hot Tub?


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.



Each day we get a number of customers contacting us asking for advice on how to clear a green pool. The green is caused by algae which are microscopic coloured plant life that grows very quickly and very rapidly in sunny and warm conditions. The only way you can get rid of algae once it has established is by killing it with chlorine and in some instances it can take a lot of chlorine, money and time!!

With this in mind it is prudent to do all you can to help prevent the algae growing in the first place. Obviously keeping your chlorine and pH levels within the recommended range and shocking your pool regularly, particularly in the summer, can help reduce the chances of algae growing in your pool. In addition to this you should also use an algaecide.

Commonly there are two types of algaecide available in the UK. A copper based algaecide which is a long-life algaecide that typically provides 3-6 months protection against algae from one dose. This is ideal for when you close your pool down for the winter. The disadvantages of using a copper based algaecide are that it can cause staining in pools that have a pH greater than 7.3. Bathers with bleached hair should also beware that copper based algaecides can turn the hair green! The alternative is a non-copper based algaecide which is added on a weekly basis. You should also consider using multifunctional tablets as your source of chlorine as they contain an algaecide as well as the chlorine and flocculant.

Preventative action after summer rain storms


After all the recent good weather it appears that the majority of he UK could be in store for some thunder storms. With this in mind you may be tempted to forget about your pool.  This is a mistake as pools often go off when there is rain following  good weather. The rain washes dust and algae spores into the pool and the dirt uses up the chlorine, allowing  the algae to thrive in the very warm water. The water goes green and it then requires a lot of shocking with chorine and dosing of algaecide to get the pool back to its previous pristine condition. 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 thundery 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.