Agreed! Some tap water is low and Mg! I had the same problem! Need to add a dose of Epsom salt next feed and maybe just a couple after also! Fixed it right up for me! Leaves stopped discolouring, I just added like 1/2 teaspoon with every litre of water after dissolving it in hot water
Epsom salt is 100% water soluble. It will dissolve quicker in hot water (like any other salt) but it will also dissolve into cold water as well. Typically your hot water is going to be "harder" than your cold water from the tap due to evaporation, so it's better to use the cold tap water in the garden. I do dwc and I like to add some tap water to my reservoir to help buffer the pH. The higher the tds content the less pH swings.
If you're mg deficient because of a lockout caused by an excess amount of calcium, then yes, adding cal-mag won't help as there is generally more calcium in cal mag than mg. One way to tell, as it's not terribly easy to see a calcium toxicity, is to check the medium (assuming it's not dwc), and if the dirt is overly 'loose', then you have enough or too much calcium. A medium that has a closer ration of cal:mg, will be more compact/dense as magnesium is a smaller ion than calcium and pulls your dirt closer together where calcium is larger and 'spreads' it out. With that being said, the overall ratio you want to maintain, if growing in heavy dense clay soil, is about 7:1 cal:mg. In looser/sandier soil, it can be as low as 3:1 cal:mg.
