AsNoriuanswered grow question 2 years ago even if its coco, you gave yourself answer what is wronh - PH
feeding may cause it too, but ph is WRONG 100%
Good ph and watering regime in big soil pot brings wonderful results even without feed or feeding very very little at late stages of grow.
Sort it, buy aquarium drops, they are cheapest and last long.
Ph should be last thing you adjust or add before watering.
Happy Growing !!!