Where I live I don't have much access to any of this stuff and had to order what stuff I did from a place roughly 4,000km away. So the MgSO4 was all I could get.
It's probably the hydrated form, so MgSO4.7H2O. But to make sure you can check the appearance of the salt. The grains are very bright and opaque white or it's kinda transparent?
This is the anhydrous from of MgSO4
This is the heptahydrate form of MgSO4.7H2O
Can I still use it to make my macro mix? If so how do I work out how much?
The EI dosing method puts a lot of nutrients in the water. What I suggest you to do is start dosing less than EI and increasing it after a week or two and see how plants (and algae) will react.
For example, you may start dosing little amounts every 3 days with 1 day not dosing at all in the day before the water change. Like 2,5ppm NO3 (what also add 1,6ppm K); 0,25ppm PO4 and additional 1ppm K (to make it 2,5ppm in total along NO3).
Also, I do suggest you to make separated solutions for NO3, PO4 and K, and dose it separately but in the same day. After some time, you may check for algae grown and adjust the dose accordingly. For example, if you see green spot algae after a week it may suggests that you need more PO4, sou you can dose more PO4 without increasing NO3.
I don't know the GH as I don't have a test kit for it, however I know my TDS is about 110 if that helps at all?
TDS measurements are a little bit confusing because each manufacture use different salt solutions to calibrate it and every ion dissolved into the water contributes differently to the conductivity measured by the TDS. You may check for Calcium or Magnesium deficiency symptoms in your plants and add the magnesium salt (or calcium salt) only once after each water change to correct the values and ratios. You should not dose Mg every few days like the macros.
Other important thing, the water change volume will impact directly the amount you need to add of those nutrients, so what is the volume of water change you will do?
Would calcium possibly also be in the micro mix?
Usually micro mixes contain Fe, Cl, Mn, B, Zn, Cu, Mo and maybe Va and Ni. If its contain any Ca or Mg, it would be so little in concentration that may has no effect at all. You need large amounts of Mg and Ca in water (10~30 ppm Ca, and around 5~10 ppm Mg) than just the 1 ~ 0,001 ppm micro mix would provide.
So, lets see what you could be doing about it all.
You can make the 2L solution of KNO3 and K2SO4 and set it to increase 5ppm NO3 + 5ppm K for each 75ml added.
So you should add 163.05 grams
of KNO3 and 82.45 grams
of K2SO4 to your 2L dosing container.
Add 75mL of that mix to your 750L aquarium to increase NO3 5ppm and K 5ppm.
For the second bottle, you can make 2L solution of KH2PO4 and set it to increase 0,5ppm PO4 for each 75ml added.
So you will need to add 14.33 grams
of KH2PO4 to your 2L dosing container. Add 75mL of that mix to your 750L aquarium to yield 0.5ppm PO4 and 0.21ppm K.
So your schedule could be something like this
Sat - 50% water change + 5ppm NO3 + 5ppm K + 0.5 ppm PO4 (75ml of each bottle)
Sun - micro mix
Mon - micro mix
Tue - + 2.5ppm NO3 + 2.5ppm K + 0.25 ppm PO4 (37.5ml of each bottle)
Wed - micro mix
Thu - micro mix
Fri - add nothing
Following these values , if you change 50% of your water weekly, and ignoring the uptake of your plants, macros will built a max value of 15ppm NO3, ~15ppm K and 1,5ppm PO4. If you do less volume of water change, those values may increase and you may dose less during the week.
All will depend on your water change volumes and plant needs.
Regarding magnesium, you may add Mg in new water before dip it into the tank.
So you may dissolve 38.03 grams
MgSO4.7H2O in 1L and then add it to your 750L aquarium to raise the Mg by 5ppm. You should do it only after the first water change. If you change 50% of your water, then after each water change you may add only 19.0 grams
of MgSO4.7H2O in order to keep the water with the 5ppm Mg.
Do you have a balance to weight these salts?
Bump: This is to make you understand a little bit more about the Rotala Calculator: