Magnesium level won't stay up

[laurentb]

Randy,
can you explain me why a use 1 bottle of kent marine Magnesium every other week ? the mag level drop constantly (from 1280 to 1150 in a week). my Cal & Alk are stable. 420 ppm cal - 9 kdh alk maintain by Kalk reactor.

[Randy Holmes-Farley]

How much limewater (or lime) gets delivered in that week?

[laurentb]

1 gal a day.

[Randy Holmes-Farley]

Did you do water changes during these periods? Have you measured magnesium in the water change water?

7 gallons of saturated limewater, if converted into typical calcium carbonate, will make about 54 grams of calcium carbonate. That might contain about 4% by weight of magnesium, or 2.2 grams (2,200 mg).

In 50 gallons (190 L) of water volume, that is 2,200/190 mg/L = 12 ppm.

I would not expect magnesium to drop more than about this value per week. Worst case, maybe twice that. But not 130 ppm.

I suspect there may be some element of testing error, as there are not other sinks for magnesium, aside from water changes. Was water changed during this period?

These articles may help:

Do-It-Yourself Magnesium Supplements for the Reef Aquarium
http://reefkeeping.com/issues/2006-07/rhf/index.php

Magnesium in Reef Aquaria
http://www.advancedaquarist.com/issues/oct2003/chem.htm

Magnesium and Strontium in Limewater
http://www.advancedaquarist.com/issues/dec2003/chem.htm

[laurentb]

yes i did a 15 gal water change using tropic marine salt. (from LFS).

[Randy Holmes-Farley]

Try measuring the magnesium in it and see if it is lower than your aquarium.

[laurentb]

ok, thanks.
i'll try that tonight.

[Randy Holmes-Farley]

You're welcome.

Let us know what you find. :)

[kwgozy]

[QUOTE]>Originally posted</a> by Randy Holmes-Farley
[B] 7 gallons of saturated limewater, if converted into typical calcium carbonate, will make about 54 grams of calcium carbonate. That might contain about 4% by weight of magnesium, or 2.2 grams (2,200 mg).

In 50 gallons (190 L) of water volume, that is 2,200/190 mg/L = 12 ppm.

I would not expect magnesium to drop more than about this value per week. Worst case, maybe twice that. But not 130 ppm.[B][QUOTE]

What is the method of calculation you used to get these results, or formula used? If not to complicated or time consuming to explain. I've been trying to research and learn some basic to advanced chemistry, thinking about a class.

Thanks Kyle

[Randy Holmes-Farley]

Sure. There are a number of ways to do it.

Saturated limewater contains about 0.0204 moles/L of calcium (or 810 mg/L).

So 7 gallons contains 810 mg/l * 7 * 3.8 L/gallon = 21,500 mg = 21.5 grams.

Calcium carbonate is 40% by weight calcium, so that corresponds to 54 grams of calcium carbonate.

Coral skeletons contain up to about 4 percent magnesium by weight (coralline seems to top the lists at 4.4%, some are much lower, less than 1%):

Magnesium in Reef Aquaria
http://www.advancedaquarist.com/issues/oct2003/chem.htm

so 4% of 54 grams is 2.2 grams of magnesium (= 2,200 mg)

In 50 gallons (190 L), that corresponds to 22,000/190 = 11.6 mg/L = 11.6 ppm.

Make sense?