Help with diagnosis

This is my first grow and all has gone well until 5 weeks into bloom cycle. Hydro/Indoors.
Yesterday I noticed some brown spots on the leaves in the canopy, not burned leaves as no height change in the last couple of weeks. Tonight is it was many more leaves and the entire leaf. The leaves have started to curl and again this is confined to the canopy. And it is confined to leaves beneath the lights. My mentor suggests it is a neut overdose, but if that is true why is it just appearing just under the lights and only the canopy. If it is an overdose on neuts should I flush immediately. I will need to water again tomorrow.

5 weeks in flower and under the canopy sound pretty normal. the plant is using itsself up like it should. if you don't see any problems on the bud site I wouldn't worry. how long till you are thinking about harvesting?what's your ppm level at?

2-3 weeks, buds look really good but smallish. ppm is 1640, high, ph is about 6.1. I am thinking of draining my res about half and adding water and ph-ing to cut the ppm. I am supposed to water tonight when the lights are on but may wait another day. Does the cutting the ppm method above sound like a solid idea?