AsNoriuanswered grow question 7 months ago Its magnesium. Cheapest fastest way - epsom salt , aka magnesium sulfate .
CalMag depends on brands , some work good, some dont. My best testested was TNC CalMag. Small doses and great result always.
I found corelation with overwatering and magnesium issues , make sure you have dry/wet cycle. Roots needs it for getting air, soil needs ot to prevent ph fluctuation .