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What is it?

TDS or Total Dissolved Solids. This refers to the amount of dissolved ions, minerals, salts, metals and other chemicals in water. The amount is expressed in mg/L or parts per million (ppm).

Pure water has virtually no conductivity of electricity. But the more TDS rises in water the greater the conductivity of the water becomes to electricity and so the value of TDS can be measured and expressed by a electronic device (called a TDS or EC (Electrical conductivity) Meter) which passes a minute current through it and gives you a readout which is in mg/L or ppm. This reading is directly related to the purity of the water. So the higher the reading the more solids are dissolved in the water.

Aquarium use

Often the TDS reading is taken by scientist discovering a new species of fish in the wild and wanting to know what kind of water the animal lives in. In general aquarium use it is rarely used by most aquarists. However taking a small sample of the water (20ml) and performing a TDS reading does allow the serious aquarist to measure the level of hardness and overall quality of the water.

A high level of TDS may indicate lots of organic material in the water and a lower ability of the water to hold oxygen. It is often used to indicate a water change is required even if Nitrate levels are low.

In a coral dominant marine aquarium, water with less than 10 mg/L TDS should be used for evaporation top-off water. For water changes less than 5 mg/L TDS should be found within the water before mixing in salt. Allow the salt mix to determine the chemistry of the water. While products such as Seachem Prime will make the water safe for use in a fish only marine aquarium, the use of straight conditioned tap water may lead to the accumulation of phosphate, ammonia, nitrite and nitrate if the tank isn't cycled. Which will harm coral, and promote nuisances such as cyanobacteria, bryopsis, and diatoms. Such water can be purchased at many local marine aquarium stores, and if none is available inexpensive RO/DI units may be purchased over the Internet.

TDS in mg/L
0 50 100 200 300 400 500+
0-50 from RO, distilled or deionised water (Very soft)
50-150 water from carbon filtration, mountain springs (Soft)
150-300 typical human taste acceptable tap water (Medium to hard)
300-500 Unpleasant tasting hard water (Hard to very hard)
500+ Unacceptable for human consumption (Liquid rock)
*See GH for more on General Hardness details