are you using soil?

Mg deficiencies are noticable from the tips of the the leaves, they die and usually curl upwards. Purple colour can sometimes build in the stems and petioles.. you'll usually notice a Mg problem at the top of the plant first.. the growing shoots go lime coloured. In some cases white with green veins...kinda creepy really.. they they saw a ghost

Treat Mg symptoms with one-half teaspoon of epsom salt to each Quart of water, water as usual.

if you're running soilless, then chances are you should be getting the plant Mg through some other form of fertilizer

it's possible it's not an Mg problem.