Magnesium dosing calculation (weight base) for small tanks

[snowdrop6]

Hello,

I recently decided to dose Mg in my Fluval Edge 6 gallon tank. Instead using a commercial product, I'd like to use DIY supplements. I found a lot of useful information (mostly from Randy's recipe); however, I had hard time to understand the details of the calculations. I don't think I'm the only one.

So I wanted to add a little bit more information here, especially for reefers having a small tank.

While for a big tank, volume-based calculation is easy, for a small tank, I think weight-based calculation is much better. So I'll show how to get the proper weight of the supplements to mix. You can also convert it to the volume base if you know density of the chemicals (gram/volume). Easy way of doing it is that take a cup of chemical and measure how much it weighs.


Here is the calculation.
- molecular weight of Mg: 24.305 g/mol
- molecular weight of MgCl2.6H2O (magnesium chloride hexahydrate): 203.303 g/mol
- molecular weight of MgSO4 (magnesium sulfate anhydrous): 120.366 g/mol

- Mg weight ratio in MgCl2.6H2O: 24.305/203.303=0.1196
- Mg weight ratio in MgSO4: 24.305/120.366=0.2019

- 1 gallon water = 3,785.41 ml ~= 3,785.41 g
- Mg weight for 1 ppm in 1 gallon water (0.001 g/1,000 ml): 3,785.41x0.0000001 =0.00378541 g

We will use 10:1 mixing ratio for MgCl2.6H2O and MgSO4. You can change the ratio if you want and change the calculation below.

- Mg weight for 0.9 ppm in 1 G water with MgCl2.6H2O: 0.9x0.00378541 = 0.003406869 g
- Mg weight for 0.1 ppm in 1 G water with MgSO4: 0.1x0.00378541 = 0.000378541 g

- Actual MgCl2.6H2O weight needed for 0.9 ppm in 1 G water: 0.003406869/0.1196 =0.0285 g
- Actual MgSO4 weight needed for 0.1 ppm in 1 G water: 0.000378541/0.2019 =0.001875 g


So using these values, we can calculate how much MgCl2.6H2O and MgSO4 are needed to increase A ppm in B G water by multiplying A and B.

- MgCl2.6H2O: AxBx0.0285
- MgSO4: AxBx0.001875

For example, to increase 35 ppm (from 1245 to 1280 ppm) in my 6 G tank:

- MgCl2.6H2O: 35 x 0.0285 x 6 = 5.99 g
- MgSO4: 35 x 0.001875 x 6 = 0.39 g

Another example, to increse 100 ppm in 10 G tank:

- MgCl2.6H2O: 100 x 0.0285 x 10 = 28.5 g
- MgSO4: 100 x 0.001875 x 10 = 1.88 g


Just enough amount of water can be used to dissolve both chemicals. In case of the first example, 100 mL water might be enough for such tiny amount.

Let me know if there is anything wrong with the calculation. I'll correct it.
I also attached an excel file. I hope this benefit other reefers.

By the way, sorry about messy significant figures.

 

[bertoni]

Most people are using epsom salt for the magnesium sulfate, which is a heptahydrate. What are you using?

 

[snowdrop6]

Magnesium dosing calculation (weight base) for small tanks

I'm using anhydrous form from chemistry supply. In case using heptahydrate, we can add molar weight of 7H2O which is 18.015x7=126.105 g/mol.

I'll add that case in the excel file later tonight.


Sent from my iPhone using Tapatalk

 

[snowdrop6]

For Epsom salt (heptahydrate form), it turns out that we can add MgSO4.7H2O approximately twice more than anydrous form. Attached find an excel file with update.