Thats a nice black pussy you have there BB in you ava

Well that does sound like a strong feed. If there is no problem above the grow medium, there is no problem in the grow medium. I think when you harvest you should post a pic of two of the root structure. The root structure is likely do be clutching the coco as to hold the plant up, probly why the medium is contracting and exposing the roots. Doesn't sound serious, not in them 15-20L pots any how!