How to Figure Out gH by Ca and Mg Levels? - The Planted Tank Forum
Old 11-19-2005, 07:48 AM Thread Starter
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How to Figure Out gH by Ca and Mg Levels?

Well I just wanted to know if it is possible to find out general hardness by measuring Ca and Mg levels? What is the general formula?
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Old 11-19-2005, 11:45 AM
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in FW, there really isn't a measuring kit to give you a good reading of Ca or Mg, so I will assume that you mean how to tell what GH will be when dosing Ca and Mg products:

The general formula is:

Ca in ppm x 2.5* + Mg in ppm x 4.1 = actual GH ppm

*the x2.5 factor is only relevant if dosing any product other than CaCO3 (calcium carbonate). If dosing CaCO3, then you do not have to multiple by anything (i.e. x1).

I believe you can use this same math on a city water report if they list Ca and Mg in ppm there. They generally tell you if the Ca content is from CaCO3 or not.

Cliff
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Old 11-19-2005, 07:17 PM Thread Starter
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Well if I cannot measure the gH using Mg/Ca test kits, what accurate gH test kit do you recommend?

Quote:
Originally Posted by Spar
I believe you can use this same math on a city water report if they list Ca and Mg in ppm there. They generally tell you if the Ca content is from CaCO3 or not.
The problem is the last water report our city has posted was back in 2003. And it states that the pH is 8.1, while I got 7.6 (+/-0.02). And it claims the kH/Alkanity is 164ppm while I currently have 2meq/L (Salifert). I believe the water has changed considerably since then.
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Old 11-20-2005, 12:09 AM
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Most GH test kits (as I understand it) are EDTA chelation tests. Calcium and Magnesium cations react equally using these tests. If you want to get an approximation of what the magnesium levels are, you can do a calcium test as well - the difference would be the magnesium levels.

For simple GH tests, the Red Sea or Aquarium Pharmaceuticals kits should be fine. They are a little simplier as they use one reagent. Many people consider the Lamotte kits to be more accurate. The Lamotte kits use two reagents.
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Old 11-20-2005, 06:26 AM Thread Starter
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Quote:
Originally Posted by Hypancistrus
If you want to get an approximation of what the magnesium levels are, you can do a calcium test as well - the difference would be the magnesium levels.
I don't exactlly get you what you're trying to say. Are you saying that I can get the gH by adding Ca and Mg ppm levels together?
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Old 11-20-2005, 11:08 AM
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Yes, GH or total hardness is primarily a measure of calcium and magnesium ions.
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