Aquatic Plant Central - View Single Post - Understanding GH & TDS & Hardness
View Single Post
Old 03-19-2010, 06:49 PM   #7 (permalink)
Senior Member
Join Date: Feb 2010
Location: North Carolina
Posts: 170
iTrader Ratings: 0
goldier is a regular member
Default Re: Understanding GH & TDS & Hardness

Originally Posted by Diana K View Post
It is not so simple as just adding things up. The GH test for example compares the things it is testing to calcium carbonate.
Similarly the Ca and Mg tests may not report Ca and Mg, but some other equivilant. (Sorry for being so vague, it has been a while since I have looked into these details)

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 View Post
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.
goldier is offline   Reply With Quote