Total dissolved solids is a measurement of everything that has ever dissolved in your swimming pool water. This includes minerals that separate from your pool chemicals, such as calcium and sodium, and every bit of dissolved dust, pollen, swimmer waste, algae remains – everything!

TDS naturally builds up in swimming pools, and in commercial pools using liquid chlorine, TDS can raise by several thousand ppm per year. In covered pools or indoor pools, which see no rain and little evaporation, dilution occurs very slowly. Outdoor pools with sand or DE filters that are backwashed frequently, or snowbelt pools that lower the water level for winter have more fresh water added to the pool to dilute the TDS level.

Distilled water has a TDS level of 0, drinking water has a TDS of under 500 ppm, and sea water can be as high as 35000, but what about pool water?

There are varying opinions on what level of total dissolved solids is too high for swimming pools. Some say that a level of 1500 ppm or above is too high, and some say that pools can operate without problems up to 5000 ppm, while others say that TDS can be even higher.

Salt pools, those that use salt chlorine generators, routinely operate with a salt level of over 3500 ppm, plus the calcium and other dissolved bits in the water, can easily take the average salt pool over 5000 ppm. At which point, the water will feel oily and taste a bit salty.

It could be, but in most cases it is not. It depends on what constituent particles are making up the high TDS level. Chlorides and sulfates can cause the most problems, reducing chlorine efficacy, giving water a dull appearance and allowing algae to bloom more easily. Also contained in high levels of TDS are usually high levels of phosphates and nitrates, which can consume chlorine and provide nutrients to algae.

High TDS may be a problem if you are having repeated issues with maintaining chlorine levels and clear water – even though the water is balanced and over-filtered. High TDS levels can also cause low-grade corrosion to metal pipes, ladders and lights in a pool, due to an adjuvant increase in the pool water conductivity.

It is a generally accepted ‘fact’ that high TDS levels suppress the activity of free chlorine. It is not known exactly how, but one theory is that a high level of dissolved solids fills the water with solids that inhibit the movement of sanitizers. Another theory proposes that the particles contained in the TDS measurement are bonding to the chlorine molecule and disrupting normal behavior. Another possibility is that high TDS interferes with testing methods, producing false (low) results during chlorine tests.

There is no chemical to lower TDS levels in pools, and like cyanuric acid, the only way to lower the level is by dilution, or replacing some portion of pool water with fresh, unsaturated water. But wait, before you start draining the pool, even if your TDS level is high and some pool store test jockey tells you to drain it, make sure that other adjuvant issues are not causing your pool problems.

-- Pool filtration and circulation changes over time. Check that your filtration and flow rates are optimal and effective.

-- Pool water balance. High pH, high alkalinity, high calcium hardness, high cyanuric acid

– can all cause dull, cloudy water with increased sanitizer consumption.

-- Phosphates and nitrates also consume sanitizer and cause adverse water conditions. You can test for phosphates with a test strip, and use Phosphate remover if detected.

It depends on what the high TDS is made up of – if the constituents are phosphates, nitrates, sulfates or chlorides, it can cause an array of water issues. Unfortunately, testing methods for pool owners and pool stores are not that sophisticated, and cannot break down TDS into its constituent components.

In most cases, balancing the pool water, treating for phosphates or changing filter media can solve the problem – but if not, replacing some of the water to reduce the TDS level should ‘fix the water’.

-- Cloudy, dull water (and your filtration and circulation and water balance is proper)

-- Trouble keeping a chlorine level overnight after shocking (and cya level is under 50 ppm).

-- Trouble with algae (and phosphate level is less than 1000 ppb).

-- Trouble with scale (and calcium hardness level is under 400 ppm).

-- Salty tasting water that leaves crusty deposits as it evaporates on deck.

When everything above has been double checked as correct regarding filtration, circulation, sanitation and phosphates, but you still have trouble with scale, algae or chlorine levels, test the TDS. If levels are above 2500 ppm, consider draining some or all of the pool water and refilling with fresh water of a lower TDS level (hint: test your fill water).

Testing for TDS in pools is best checked for with a digital test meter that measures the conductivity of the water to more accurately detect dissolved particles. Aquachek makes a TDS test strip which should be accurate enough for most applications.

Chlorine

Cyanuric Acid

Digital TDS Tester

Calcium Hardness

Phosphate Remover