Ah right. Could this be why some people don't have luck with EI? Maybe they needed magnesium?
Well, some may blame the symptoms of Mg deficiency on EI, that certainly happens, however, using an EI dosing strategy does not guarantee overall success.
Hobbyists fail every day and just because they lay blame at the feet of some factor it does not mean that that factor is the culprit.
There is a lot more to plant husbandry than just the dosing program. What about poor CO2? How about poor flow/distribution? What if the hobbyist has poor maintenance habits?
Lots of bad habits, misdiagnoses of faults, as well as misunderstanding of basic principles can lead to failure with planted tanks.
EI is not responsible for stupidity, stubbornness, or ineptitude, yet, many people blame their failure on this dosing program while refusing to look in the mirror.
Nutrition is only one facet of the game.
It's also important to reiterate that Calcium and Magnesium are mirconutrients. I can't recall how many times I've had to repeat this mantra. Calcium and Magnesium only require a non-zero value to be present in the water column. I've run tanks using RO water adjusted only minimally to 2-4 dkH using products such as Seachem Equilibrium or the Sera product. Small amounts were added at water change and the pants were fine.
Micronutrients are not mobile in the plant. The stay in their originally distributed location and the plant continually accrues their concentration in each leaf.
That's why micronutrient shortages are seen primarily in young leaves, which have not had enough time to accrue sufficient quantities.
So you see, it doesn't really matter if the waters concentrations of Ca and Mg are low because as long as they do not fall to (and stay at) zero, each leaf will continue to absorb them and over time the concentrations within the leaf will rise, so this is self regulating.
In any case, Ca and Mg deficiencies will not cause catastrophic failure in a tank. I don't know why these two minor elements somehow morph into flavors of the month.
If plants fall over an melt into oblivion, it will never be due to shortages of these. Catastrophic failure means that the hobbyist is doing something seriously wrong which has nothing to do with Ca or Mg.
Cheers,