Might be nute burn ----
check out this guide
https://www.rollitup.org/general-marijuana-growing/420048-nute-burn-deficiency-guide-diagnosing.html
Also, nute burn can result in your PH being off as only some nutes are absorbed at different levels. 5.5 for the water is good. should check out your soil
Yes, as well as it might be underferting (lack of nutrients there of). Especially because he then said:
Coco soil I think not quite sure got pot full off a friend :/ well what sort of fert shall I use can you use hydro feeds in soil?
Coco does not contain nutes, therefore it needs some. Being mixed with soil, it was good enough for the seedlings, because it provided just enough food, and also kept it a long time in veg already, so you see my point above. And 5.5 is NOT okay for coco/soil mix, it is ok for coco ALONE; it should be the same as for soil: 6.2 - 6.8 optimal levels. In fact, it might just be what's causing a lockout, so stop talking nonsense.
Now, Jwee: you need to tell us:
- if you've been using any nutrients so far; you mentioned you have some a/b; what brand is it? there are many a/b's out there...
- what are the percentages of coco and soil that are mixed together (it IS mixed with soil you said, right?)
- what's the ph of the runoff? The water itself is not okay so far, but endulge us.
- is the yellowing progressing from the bottom up? (this would be a Nitrogen deficit)
- has the yellowing occured on the newest growth only? (Sulphur def.)
For the last 2 questions, you could refer to the list of deficiencies listed above by solarphlare and also go through the rest of them in that list that could be related to yellowing, I just started you on.
There, after this we should have a better understanding of the situation. Crazy mumbo jumbo.