You are correct blackomne and Nas19320
Measuring parts per million ppm TDS Total Dissolved Solids is just another way to describe how much "stuff" is dissolved in the water.
TDS measured by how conductive the liquid is.
Conductive meaning the liquid will pass an electrical current.
A liquid is conductive when it has has ions.
Pure water ( very pure like deionized water ) is a horrible conductor of electricity.
There are very few, if any, ions in the water, so electricity will not pass easily through this water. The pure water is actually resistive to the current, and acts as an insulator.
When solids like salt and minerals are dissolved ( ionized ) in water - then that sample of water will conduct electricty using a conductivity / TDS meter.
As you said, we use TDS or conductivity/resistivity to measure how pure the water is coming out of our RO, DI or RO/DI filters, or tapwater if we choose to use it.
Very pure water will have a very low ppm TDS reading.
Very pure water will have a very low conductivity reading ( microSiemans µS )
1 µS microSieman is roughly .5 TDS
Tapwater would be considered somewhere in the 50 to 1000 µS
or 25 to 500 ppm TDS ( this can vary a lot around the world depending on the municipality )
Seawater on the other hand is somewhere around 70,000 µS or close to 35,000 ppm TDS
The instruments we buy for measuring this stuff are all conductivity measuring devices.
If we know the conductivity - we can infer the ppm TDS.
If we know the conductivity - we can infer the salinity.
If we know the conductivty and temperature - we can infer the specific gravity.
Once you properly mix the salt into the pure water - you'll have your saltwater for the tank.
It's salinity ( specific gravity ) will affect it's conductivty ( TDS ) reading.
There are no ill affect of having a ppm TDS reading in the normal seawater range in our marine tanks.
But as time goes on - if you do not regularly top off with freshwater, or stuff in the tank were to slowly begin to dissolve - you could in theory have a rising ppm TDS - and that over time could harm the tankwater chemistry and creatures.
In addition to evaporation and stuff dissolving in the tankwater, dosing additives and using a protein skimmer can also increase the TDS over time.
Here's the biggest problem though - and it deals with using tapwater for evaporated top off water.
Over time - as you add this tapwater that contains minerals and dissolved solids - your TDS will rise. It has to.
The tapwater contains ionized minerals that do not evaporate.
Over time - these minerals will continue to rise in concentration.
If you do regular water changes - you'll keep things in check.
If you are one that does not believe in doing water changes - and you continue to use tapwater that has a very high TDS reading - you best be checking your specific gravity / salinity often - and be prepared to take action.
That action is a partial water change. Period.
