Even in approximation, they do add up in simple arithmetic fashion either in TDS or the general GH because ppm unit is the same as mg/L or g/1000000 mL (part per million). The cations Ca++ & Mg++ each has a specific weight of their own. Of course test kits canít measure the weight of each cation, so they sense and approximate the total number of the cations in a finite volume of water, then interpret the result in total weight per water volume expressed in ppm or in degree. By the same token, the test kits can also use the anions numbers (bicarbonate, carbonate, sulfate), as each of them corresponds to a cation Ca++ or Mg++ into a weight value to come to the same ppm result.
Originally Posted by Diana K
There are not many plants that will thrive when the water is so low in minerals as to measure just 100 ppm TDS. Most plants that we keep in aquariums come from somewhat harder water than that, or, even if they come from soft water, they can thrive in harder water just fine.
Itís clear to me now that there is no correlation of hardnesses between TDS and GH. GH mainly refers to the measurement of Ca & Mg only, while TDS includes many more things in the water. I didnít pay much attention to TDS before, but this thread certainly gives me an incentive to own a TDS meter to know which electrolyte is exceedingly in excess or lacking to the detriment of plants or fish health. Thanks.