I hope youre not letting that coco dry out. Coco needs to be constantly wet (plant has to have an established root system of course). If you let it get dry the fertilizer builds up in the coco causing nutrient lockout and worse burn from excessive nutrients. What I would do is check runoff ppm/ec and if its higher than what you feed (by 300ppms), then you have a salt buildup. If its lower than what you feed with, then increase the feed. I would start with a runoff ppm/ec check, if its over your input (by 300), flush with a 300 ppm( Im using the 700 scale/canna nutrients) of base nutes, I would do calmag flush since coco hogs it. Then I would probably feed at 500ppm base nutes (Remember this is what I do if its 300 ppms over my input). Good luck!