Thx polish but I do use a ppm meter. Aka EC meter. My ec is at about 1.7-2.0 when the plants are fully matured, from about 4th week of flower onwards. My guy at the hydro store suggests these figures. I suppose I could lower the nute levels.....still sorta in the dark here. On my newest babies I bought some calmag+ and I've been changing the Rez every 10 days and closely monitoring ph and just a few days ago I noticed early signs of this same problem. It begins with very small brown dots speckled all over the fan leaf, and a burnt tip. Soon it will probably spread. I'll take pics tonight to show u what it looks like in the early stages.