Actually the egg shells and vinegar makes a calcium acetate. Roasting the eggshells first adds a bit of phosphorus to it.
Most CalMag is just Calcium Nitrate and Magnesium Sulfate. I’ve used General Hydroponics CALiMAGic in the passed with good results. I wouldn’t suggest an organic type calmag product in a resivoir. I get bacteria blooms when I did.