Pale plants, magnesium response & feeding question

Big plant still shows magnesium issues, what for you wait ? Why you cured smaller one only ?

side note... i bet the "2 colors" you seein the leaves is more about the ripple and photography.. if oyu flatten it out, it's probably 1 solid green or at least not as profound as it looks in the pictures. sometimes the photograph can exagerrate things... just as intense light can ... though the latter can help you notice slight chlorosis sooner, in my experience.

light mix comes with a very low charge of nutes, like promix. it's not meant to last long. You treat it like a soilless medium after that... it needs a full diet at all times. i'd suggest 75ppm of Mg - cumulative total across everything you add. that is more than enough to keep the plant happy under normal circumstances. i use 1g/gal epsom but my "Part A" also has magnesium-sulfate in it.. .they add up to about 75-80 ppm.. i'd mimic the ratio of what jacks 3-2-1 provides... you may need a lower or higher overall concentration relative to local environment, but the ratios are sound as far as providing nutes around the rooots without impeding each other and safely supplying everything it needs. religiously get 10% runoff, and those nutrients around roots remain consistent over time. Any issues you see related to nutrition is fixed through adjusting formula... easy-peasy. the other stuff has way too many variables... you can't assume all the differences between two seeds is caused by you... the fact tehy are in different sized pots probably had a more profund effect on size difference than any minor nutritional issue. Genetic variety could cause it even if all other factors were equal.

If ph is ok then there hungry. Nitrogen mainly needed and Epson salts for the Magnesium deficiency