Freshwater magnesium (Mg) test kit?

[AquaPop]

All of the Mg test kits available seem to be exclusively for use in marine tanks.

Does any one know how to test magnesium levels in fresh water?

[Donny@ageofaquariums]

There are......... but you may find this an easier way, even though it does seem complicated...... it only really is the first time.

You need to know your GH and Ca to know your Mg levels. The last part of this note speaks of the hard water areas.

CALCULATING Ca and Mg from GH

In order to find magnesium hardness, you must take a hardness reading with a GH test.

Lamotte's calcium test is a GH test with an inhibiting agent that prevents magnesium from affecting the test. There's no way you can find the magnesium level by using the calcium test alone.

The calcium reading of the calcium test is in a CaCO3 equivalence. CaCO3 is 40.04320% calcium. So if the kit reads 75 ppm as CaCO3,

then 75 * .4004320 = 30.0324 ppm actual calcium.

For magnesium, take your GH reading minus your calcium reading. For example, if your GH reading is 100 ppm as CaCO3, then 100 - 75 = 25 ppm magnesium as CaCO3. If you want to express this as actual magnesium, multiply by .2428391. So 25 * .2428391 = 6.0709775 ppm actual magnesium.

(In case you're interested, the .2428391 comes from the assumption that calcium and magnesium react equally on the GH test. Calcium weighs 40.078 grams per mole; magnesium weighs 24.305 grams per mole. Therefore, calcium weighs 1.64896 times more than magnesium (40.078 / 24.305). Take your 40.04320% calcium in calcium carbonate, and divide by 1.64896 = 24.28391%)

Originally posted by Left C

TLDR -> General Hardness mg/L (CaCO3) - Calcium Hardness mg/L (CaCO3) = Magnesium Hardness mg/L (CaCO3)

[AquaPop]

Great info. Thanks.

Now I just have to find a Lamotte's calcium test kit. Just did a quick search and found plenty of them available for testing your swimming pool. I'll keep looking.