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3. The Magnesium stock solution and how much to dose daily

Lets assume our 250litre tank uses 5ppm magnesium daily.

Chemicals required : MgCl2.6H2O (Magnesium Chloride Hexahydrate) This is magnesium Chloride with 6 water molecules attached

MgSO4.7H2O (Magnesium sulphate Heptahydrate) This is magnesium sulphate with 7 water molecules attached, also called Epsome salt.

Source of Chemicals – Online (Buy food grade) you may be able to get Epsom salts from a chemist. Make sure you get the correct Amount of water molecules attached to each compound.

Either of these chemicals can be used to raise magnesium so using our calculator to raise our magnesium by 5ppm we need EITHER

10.4g MgCl2.6H2O (Magnesium Chloride)


12.3g MgSO4.7H2O (Magnesium Sulphate)

However as we are already dosing chemicals for alk and calcium, using just one of the above compounds will overtime cause a skew in our tank of either Chloride or Sulphate ions. We don’t want this so we need to use a mix of the above 2 compounds.

I won’t go into why but if you are using the alkalinity and calcium recipes I gave above you need to dose Magnesium Chloride and Magnesium Sulphate in a ratio of 5:3

So let’s scale the above weights in a ratio of 5:3

MgCl2.6H2O = 10.4 x 5/8 = 6.5g

MgSO4.7H2O = 12.3x 3/8 =4.6g

Lets scale these mass up now and get to a reasonable amount to add to our litre of water for our stock solution.

An Acceptable amount of stock solution to dose daily would be 50ml. So if we decide to dose 50ml of solution daily, that 50ml must contain 6.5g MgCl2.6H2O and 4.6g MgSO4.7H2O

So there are 20 daily doses of 50ml in a 1 litre stock, this is therefore our scale factor.

Now lets scale our mass of MgCL2.6H2O and MgSO4.7H2O up

MgCl2.6H2O = 6.5×20 =130g

MgSO4.7H2O = 4.6x 20 = 92g

So from the above we make up our magnesium stock solution. Dissolve 130g MgCl2.6H2O and 92g MgSO4.7H2O in 1 litre of Ro/di water

50ml of this above solution will raise the magnesium in our 250litre tank by 5ppm.

But Wait, My tank isn’t 250 litres like the above 3 examples

Simply use the calculator and enter your tank volume in litres into it, select the chemical you are adding and the amount to raise daily. Then substitute the weight given into the above working.

Right so I have my stock solutions in my containers and my doser connected up, I just select a time to dose for each container and click go yes?

Not quite 2 more things.

First you need to calibrate each dosing pump (This will be in the instructions) basically just hold the manual dose button for a specific pump until liquid starts tome come out of the end going into the tank/sump. Now take a container of known volume (ideally a measuring cylinder) – usually 100ml container, but the manual will tell you. Now select calibrate from the menu on the doser. The doser will start pumping liquid and ask you to press the stop button once the set volume has been dosed. Job done that pump is now calibrated. Now do the same for each pump.

Finally you need to set the time and amount each pump will dose. In our examples above lets say we need 50ml of magnesium solution a day. Rather than selecting dose 50ml all at once split it between say 5 doses in a 24 hour period, this helps swings and stability. When you set the times for the other solutions to be dosed try and stagger it and keep doses from different solutions an hour or so apart, this reduces the risk of any chemical reactions amongst compounds you are dosing.

That’s it you are done now, go and have a beer, you deserve one!