Ok the Starting water is very good at <50 PPM of Calcium. I would just not add any more because the Canna Coco should be sufficient. Do a gentle top fertigation flush with 30% strength Canna Coco nutrients PH to 5.8. Run-off to about 30% to waste - three times in a row over the next 3 fertigations. Then go back to your regular auto-pot with about 60% strength Canna Coco. PH 5.8.
Ps. we are only talking about flushing the one plant the others are fine just stop using cal-mag. and maybe a little less fertilizer.