How does water hardness and total dissolved solids affect my pool?



The same problems associated with hard water in household pipes, namely scale buildup, can be magnified many times in a swimming pool environment. Once hardness levels become high, scale may form on the pool tile, the pool surface, and even the pipes and equipment. Unfortunately, the causes of hard water in swimming pools are, for the most part, unavoidable. To begin with, the tap water used to fill a swimming pool contains a certain amount of calcium, the main component of hard water. It is the calcium in the water, combined with alkaline particles, which produces the visible scale buildup. Usually, the concentration of calcium in the tap water is well within, or even below, the recommended levels for swimming pools, but as the water in the pool evaporates, the calcium is left behind, since it is much heavier than the water itself. As the pool is being refilled, more calcium is being added through the tap water, and thus the concentration of calcium in the swimming pool increases.

The level of total dissolved solids (TDS) in the pool is not as important as that of calcium hardness. TDS is usually used as a tool to determine the general condition of the pool water in terms of how many dissolved particles are present. As the water remains in the swimming pool for extended periods of time, particles accumulate and eventually become so plentiful as to interfere with the function of the pool's chemicals, as well as to cause significant loss in water clarity.


Water Hardness: Water hardness is a measure of the concentration of minerals, primarily Calcium, present in the pool water. Water hardness is also referred to as calcium hardness, and at high levels, can cause scale buildup.

Total Dissolved Solids (TDS): TDS is a measure of all of the dissolved particles in the swimming pool water. These include calcium, manganese, copper, iron, other trace metals, total alkalinity, conditioner, and all substances not totally broken down by the pool water. A tool is generally used to determine the overall condition of the water and to determine whether or not the water should be removed and replaced with fresher tap water.


The Water Hardness And TDS Scales

Water hardness and ms are both measured in parts per million (ppm). Both scales begin at zero ppm and have no upper limit. Generally, the maximum levels conceming swimming pools are on the order of 1000 ppm in the case of water hardness, and 2000 in the case of TDS.

Ideal Levels

The recommended ideal levels of water hardness and TDS are 200-400 ppm and 400- 1500 ppm, respectively. The lower limit for water hardness is necessary to ensure the longevity of pool plaster and any metal components in direct contact with the pool water. Since pool plaster is composed partly of calcium, it is possible for soft pool water to pull the calcium from the plaster, causing degradation of the pool surface. Other pool surfaces, such as fiberglass, also contain some metals in them, and these metals would be removed in the presence of soft pool water. For these reasons, it is not recommended to add soft water in large amounts to the swimming pool. Concentrations of calcium higher than 400 ppm can cause scale buildup along the tile line, and on the surface of the pool itself. These scale/deposits begin to appear as simply a rough surface finish, but as they progress, they darken to eventually resemble a brownish stain with a rough texture. Low TDS levels are usually present only in soft water, andare not a major concem in ordinary tap water. It is not possible to have a lower TDS level than that of calcium, since TDS is partly composed of calcium, thus as long as the calcium is within the recommended ideal levels, the IDS should follow suit. High TDS levels, on the other hand, may cause a lack of water clarity, even if the calcium concentration is within ideal levels. If persistent cloudiness of the pool water is observed, the TDS level should be tested as soon as possible.

Time And Frequency Of Testing

Since the evaporation rate is highest during the summer months, it is advisable to have the water hardness and TDS levels tested twice per month during this time. During the Off-season, testing for these levels once per month is sufficient. There is no specific time of day to test for water hardness or TDS.


Type Of Chlorine Used

Some types of chlorine contain relatively high levels of hi-products, including calcium. It is advisable to check the ingredients for the presence of such by-products and avoid them whenever possible. It is always best to use chlorines possessing high amounts of available chlorine, that is, the actual amount of usable chlorine in the container. The highest amount of available chlorine is usually 89%, found in most tablets and some granular chlorines.

Type Of Filter Used

Some filters, such as sand and diatomaceous earth (D.E.), have the ability to be backwashed, or cleaned, using the pool's existing pwnp. During this process, some of the pool's water is lost, and as a result, so are some of the calcium and other minerals in the pool water. When done frequently, this can help to keep the water hardness and ms levels within ideal levels. Other filters, such as cartridge filters, do not use this method of cleaning, and thus the pool water is not cleansed of minerals as it is with sand and D.E. filters. In this case, water hardness and TDS levels can climb at a more rapid rate.

Dirt And Debris

Since the dirt in the southwest deserts is rich-in calcium and other minerals, the presence of it in the swimming pool can introduce these minerals to the pool water. Care should be taken to ensure that as little dirt and debris as possible enter the pool water.


The presence of metals such as copper and iron can increase the TDS level of the pool water. If large amounts of these metals are entering the swimming pool, steps should be taken to eliminate their source.