I have a magnesium deficiency some leaves are about to turn yellow, you advise me to wash the substrate with water at ph 6 and then put calmag or calmag I add it to i fert ??
Your plant does not have a magnesium deficiency.
This is what Magnesium deficiency looks like.
https://sensiseeds.com/blog/wp-content/uploads/2018/04/1-magnesium-deficiency.jpg