Welcome BillHi I hope my suggestion can help you. I have seen this before with the same leaf indication. I solved the problem with Epsom Salts I used 1 tablespoon per 10L of water. I think it is a sulphur deficiency as epsom salts are a sulphate. It cured my problem also I continued with the epsom salts with every watering. Within 5 days all of my leaves had gone dark again and I had no more problems for the rest of the run. Good luck I hope I helped you