Appreciate the advice everyone. Pretty sure that the crowd consensus of salt buildup is correct. I tested each plant's runoff individually this morning and although there were a couple that showed only 200-400 ppm, most of them were showing 900-1200. Guess I tested one of the hungry girls earlier. Spent the better part of the day mixing light nute res's (250 ppm) and flushing the whole garden out. Got it down to an average of 500 ppm runoff, so will have to flush some more this weekend.
Hq - I'm sure you're right. I'll go without any of the calmag in the next few feedings (including all of today's flushing) and see if Canna's AB holds true. I was just adding it as a precautionary since I kept reading all over the internet that if you use RO and coco, you MUST use calmag. If the calmag isn't needed, that'll be one less ingredient to toss in the res