Well I haven't used coco but the couple people I know who run coco tend to keep ph closer to 5.6-5.8, if you were in soil that ph would be awesome, but might be something to research/consider. The only other thing is I know when I have use tap water, I let it sit in an open container for 48-72 hours because most city add things such as chlorine to the water, which buy doing this allows the cool evaporate out of the tap water.
Definitely don't think the nutrients are the problem here, but I might consider giving it boost in the CalMag department. It appears from photos that you have a little bit of leaf curl beginning which can often be a sign of a CalMag deficiency.
My only concern about the nutes is the once every 3 days watering. There again, not familiar with coco first hand, but My plants get water maybe every 5-7 days however I grow and soil and let it get pretty dry before adding more water.
Granted it is a Plant Issue, but you may also want to throw a post into Newbie Central as that forum seems to get a lot of comments, just a thought, wouldn't hurt anything at least.