A TDS Meter indicates the Total Dissolved Solids (TDS) of a solution. In a way, you might say that a TDS meter is really a "conductivity meter", as It will measure all negative (anions) and positively charged ions (cations) that are present in water. These ions may include compounds such as sodium, magnesium, phosphate, silicate, calcium, and nitrate, to name a few.
Those of us who keep fishes that prefer water of lower pH/hardness will appreciate utilizing a TDS meter to quickly and efficiently check the quality of the water being produced by our reverse osmosis/deionization unit. If you are using a TDS or conductivity meter to monitor the performance of an RO/DI system, then the measured value should drop to at or near zero. Higher values indicate that the DI resin in the unit is becoming saturated and needs replacement. Of course, it's also useful for checking the water in your aquarium as well, and it's so easy to use (and dare we say...fun?) that you'll want to test frequently!
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