magnesium ??

A little burn. Happens to the best of us. Test your current solution ph/ppm/ec Calibrate your meters Test your solution again (how far were you off?) Adjust - Back down on ppm/ec. Hit your ph target. Test- measure for ph drift. Best of luck on your grow

It looks like tip burn which would mean too much nutrition. Like matey said quality coco comes ready to use. It should say on it.PH correct and buffered but the cheaper stuff often isn't. You shouldn't need cal-mag.Its without doubt the most over used product going. Spot on the leaf,give it cal=mag but most nutrients have plenty in and more often than not its the plant that's having an issue using it.Is your PH correct? If your ph is out then certain nutrients become less available.

here's a leaf chart https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg

maybe if it also develops spots... interveinal chlorosis without spots can be several different things. you should download a leaf symptom chart. cal mag alone cannot buffer coco. calcium nitrate is a better option -- better brands come buffered, but it's always a risk using coco. consider sphagnum peat moss with perlite as an alternative soilless substrate. you need more perlite with this base (1:1 vs 2:1 for coco) runoff pH being off is expected with runoff. what matters is consistency. Has this offset been consistent from the start? then, it's no big deal. Runoff pH correlates to root zone pH but is not exactly root zone pH. if leaf petioles are purple, but only where expsed to more intense light (i.e. mostly near the top), it is probably light-related. Despite popular belief this is typicalyl related to p-deficiency and not a mg-deficiency if it is caused by nutes. LED do this and some genetics do it on their own. If you react to it and cause a new problem, recognize that it isn't related to what you tried. try soemthing else next time. i've done this and given up on trying to prevent purple petioles.