Low Mg shows as yellowing between the veins on the oldest fan leaves first as it's a mobile nute and the plant is stealing it to use it upstairs where it's needed. Ca is immobile so when low tends to show up first in newer growth as the plant can't steal it from the older leaves. Only NPK, Mg amd Zn are mobile. Everything else is fixed in place once used so the plant needs to get those from the roots and if missing or locked out by the pH being out of whack, (generally too high), then symptoms show up first in newer growth.
Eggshell is very slow release calcium and best used in organic type grows where the soil is 'cooked' for some time before used with plants. Any decent CalMag will get Ca to the plant right away and supply a balanced amount of Mg along with it. Get one with iron as that's a common mineral that can end up in low supply too.
Epsom salts don't have any Ca so won't help with that but is a great way to get extra Mg when needed and supply lots of S which is why it should be used in later flowering to promote resin production. Very hard to overdose sulphur. Some extra Zn helps with that too so I'll grind up Zn tabs during the grow even in veg to help with all sorts of reactions going on as it acts as a catalyst to help in many areas. Most of us humans are low in Zn, Mg and many other minerals as well as are plants.
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. Thank you for your time and sharing your knowledge.