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Re: [APD] Total Dissolved Solids vs General Hardness

Date: Fri, 18 Feb 2005 08:34:00 -0500
From: "David Grim" <grim1214 at bellsouth_net>
Subject: [APD] Total Dissolved Solids vs General Hardness

Hi Everyone,
I am receiving an order of Discus in a couple days. I have kept and even
bred these fish in the past, but it has been a few years, and the business I
am getting these from uses a Total Dissolved Solids (TDS) parameter they
like to keep their water within. I'm getting a TDS meter for the fish, but
was wondering if TDS and GH are the same thing or different?

Any help is appreciated.

They are loosely related which does confuse folks, a bit. General hardness (GH) is the amount of divalent cations like Ca++, Mg++ and Fe++ disolved in the water. Since they (and their accompanying anions -- mostly carbonate and bicarbonate) typically consist of about 75% of the dissolved species in most US water, the Total Dissolved Solids (tds) are a rough indication of hardness until a lot of ions like Na+ and K+ get into the act and mess it up. [They increase tds with no effect at all on GH.]

GH and tds are certainly different, but once you know about how much of your water is as hardness (usually given as mg/L of CaCO3 on your annual water report -- same as ppm) you can usually count on the hardness tracking the tds as a pretty constant fraction. For example, in Fremont, I knew my 300 ppm tds water was about 200 ppm as GH, so I just took 2/3 of the measured tds as the GH. Divide the tds ppm by about 18 to get it in degrees of hardness.

The only exception I have encountered was when Bill Ruyle lived in RI or some such remote location. His well water had tidal ocean intrusion, so tds could be all over the map with almost no change in hardness

If I wanted 100 ppm hardness, I could just dilute my tap water 50% with RO water, and get a tds reading around 150. I then knew my GH was about 5.5 degrees (100/18).

Hope that helps more than it confuses. :-)


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