Total dissolved solids (TDS) is the sum of all materials dissolved in the water and normally runs in the range of 250 ppm and higher.
There is much discussion over what levels are considered too high, but there is no real lower limit. TDS is comprised of many different chemical compounds, which means that the issue of how much is too much actually depends more on what they consist of than how much there is. In general, when the TDS exceeds approximately 1500 ppm, problems may begin to occur.
It must be pointed out that pools whose sanitizing systems are based on chlorine or bromine generation equipment will likely have much higher TDS levels. These pools actually have salt in one form or another added to the pool. The salt used is highly soluble and does not cause the type of problems normally associated with high TDS, but it does add to the TDS level in the pool, as will any chemical. When testing water in this type of pool for TDS, the salt intentionally added to the pool needs be taken into account.
At elevated levels, TDS can lead to cloudy or hazy water, difficulty in maintaining water balance, reduction in sanitizer activity and foaming. Unfortunately, the only way to reduce TDS is to drain a portion of the water and replace it with fresh water. Sequestering agents do not help when high TDS levels are causing cloudy water.