Without knowing what your water is like, I can't say about the need for magnesium. Hard water often has enough so what water you have will help decide on the need. Hard alkaline water with plenty of calcium and magnesium and it is not needed to add more.
And it should alway be understood that what is right for one tank may not fit another so at times we need to adjust the amount we are added as we also may want to adjust some of the other points of EI dosing.
It is a good place to start and as all things, it can work better if we are able to spot places to adjust things lsightly.
I live in toronto canada, my local water got a ph of 7.5 to 8. so its consider hard water and rich in mineral. I just check with my local wal mart, they do carry epsom salt. but after I read ur reply I'm not too sure I'll need this stuff. I'll go ahead and set up a 5 gallon testing tank and see the result before applying anything to my main, thx for helping.
Start with what you have! NPK is basic plant fertilizer (Nitrogen, phosphorous and potassium) which what you have will provide. Obviously adding Magnesium, Sulfur, Iron and micro nutrients will result in better growth
do u mixed ur macro together like NPK in one bottle?
A general purpose GH Booster would also provide this, if you can't get Magnesium Sulphate specifically. Seachem Equilibrium, Ultimate GH Booster, etc. I dose GH Booster after water changes to reach my target GH and then dose NPK and Traces/Iron daily.
I dont think I need gh booster, my local tap is pretty rich in mineral and r consider high in ph at around 7.5 to 8 through out the years. I actually have to soften my water for some of my tank to trigger breeding for some species. by the way do u mix NPK together in 1 bottle or u mixed them individually each with its own bottle? sorry for such newbies question, I've never use EI before. but as u can see, when I got this big tank. continue with seachem seem not a viable option to me, it gets way too expansive.