epsom salts is magnesium sulphate, not magnesium and calcium.
supplements are really way over priced IMO.
You can easily use Dolomite lime which is essentially calcium and magnesium, but you'll need to use the finest grade available. It breaks down relatively slowly and is not immediately available to the plants. Works excellent for gaden beds, but not as well for containergardening for short lived plants.
For immediate correction of a magnesium defieciency, use Epsom salts
"Magnesium sulfate is a chemical compound containing magnesium and sulfate, with the formula MgSO4. In its hydrated form the pH is 6.0 (5.5 to 7.0). It is often encountered as the heptahydrate, MgSO4·7H2O, commonly called Epsom salt."
For immediate correction of (the more rare) calcium deficiency use calcium peroxide.
Most chemical fertilizers supply ample Mg and Ca, although some companies sell a Cal-Mag
solution separately to extract more money out of your pockets.
The vast majority of Mg deficiencies you seen on these forums is due to too low of a pH, which causes the Mg to be locked out and thus unavailable to the plant, resulting in a deficiency
Cannabis does however require a vast amount of Mg compared to other plants, so it's not a bad idea to add some epsom salts to your water every now and then.. especially if your leaves start to grow thin and ridged with a lime green colour as opposed to a darker shade.