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I recently bought a bag of magnesium sulfate (MgSO4) heptahydrate from AAAquatics and can't seem to work out how much I need to add to reach a desired PPM of Magnesium.

Considering GH test kits would only test for the magnesium in (MgSO4) heptahydrate, I must be making a mistake in my math... unless the GH kit tests for more than only magnesium... or something else is happening...

The MgSO4 heptahydrate (from AAAquatics) is composed of:

16% Magnesium

32% Sulfur

I am using RO water**

Let's say I start off with

__1g of MgSO4·7H2O and 10L of water (10 000g) (2.64172 gal)__

PPM = (Solute weight / Solution weight) * 1 000 000

PPM = (1g MgSO4·7H2O / 10 000g) * 1 000 000

PPM = 100

**PPM of MgSO4·7H2O in 10L when using 1g of MgSO4·7H2O = 100**

Out of the 100 ppm just found, 16% is Mg.

100 ppm * 0.16 =

**16 ppm of Mg in 10L when using 1g MgSO4·7H2O**

According to this result,

__if I want to reach 53.625 ppm of Mg in 10L :__

53.625 ppm of Mg / 16 ppm of Mg = 3.352

**3.352 * 1g of MgSO4·7H2O = 3.352g of MgSO4·7H2O to reach 53.625 ppm of Mg in 10L.**

Verification:

__16% of Mg in 3.352g of MgSO4·7H2O__

3.352g * 0.16 = 0.53632g of Mg

PPM = (Solute weight / Solution weight) * 1 000 000

PPM = ( 0.53632g Mg / 10 000g) * 1 000 000

PPM = 53.632 ppm

**PPM of Mg in 10L when using 0.53632g of Mg = 53.632 ppm**

TESTING

__3.352g of MgSO4·7H2O in 10L should = 53.625 ppm of Mg__

Results

This is where I run into a problem... Since a GH kit tests mostly for Magnesium and Calcium, I should be able to add some MgSO4·7H2O to my RO water and have that GH kit tests only for the Mg, since it cannot test for anything else in MgSO4·7H2O.

That RO water was tested prior to adding the MgSO4·7H2O and showed:

0 TDS and 0 GH

When adding

__3.352g of MgSO4·7H2O in 10L (after 12 hours) which should = 53.625 ppm of Mg__:

My GH test kit (from API) takes 9 drops to turn green which is = to 9 degrees or 160 ppm of Mg...

TDS = 145 ppm...

The 145-160 ppm cannot possibly be only Mg... and so what could it be? Is it the Magnesium + Sulfur??

Since 16% Mg + 32% S = 48% of the composition of MgSO4·7H2O

PPM = (3.352g MgSO4·7H2O / 10 000g) * 1 000 000

PPM = 335.2 of MgSO4·7H2O in 10L of water

335.2 * 0.48 = 160ppm of Mg and S

So my questions are:

Wouldn't the GH Kit only test for Mg in MgSO4·7H2O?

How do I calculate the exact amount of Mg in PPM if my math is wrong? I need that to reach an appropriate GH and respect a 1:3 ratio of Mg to Ca.

Is something happening that I am completely looking over?

Thanks a lot, I tried to make it clear but I understand that anything is always clear to the author...