You list DIY CO2, those smaller tips look like classic poor CO2 levels.
Mg deficency does NOT express itself in that manner, you folks are confusings some Ca issues with Mg.
I'd centainly add a little Mg in the water there, it's likely very pure with little GH or KH. But 1/4 teaspoon of MgSO4 * 7 H2O 2x a week is plenty for a 55 gal tank no matter what the light.
CaCl2 can be added for the Ca++.
SeaChem Equilbrium, if you can get it, would add K, Mg, Ca, SO4, Fe and Mn.
That would cover most of the bases and leave you with CO2, KNO3, KH2PO4 and trace dosing. If the KH is 2-3 or higher, then you are fine, if the GH is 5 or higher, the Ca is good.
Still, I think the tank has poor CO2 issues and that is evident from the algae on the background and on the plants.
Fixing the nutrients is rather simple, fixing the CO2 may be tougher, you need at least 2 x 2 liter DIY bottles going and a good reactor/method to dissolve the CO2 and mix it in the tank well.
That (improving your CO2) will help you no matter what else you do.
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