I'd stick with about 6.2 for coco. Your nutes are good. When you say 'tank water' where does it come from? Is it well, city, rain, or what? If it has a base ppm under 200, you need to add calmag. Coco grows need more Ca than other grows. Canna Coco is good but if you are running distilled or rain water, you have to add calmag anyway.
