Is this a nutrient burn?

3 weeks ago I measured 1250 ppm at the weekly rez change. FF big bloom trio. Tap water starts @ 250ppm. Was I at 1000ppm? 34 days into 12/12. I didn't get a meter until I started 12/12. The plants vegged 8 weeks and since their my first grow they're part of a steep learning curve. 20L/min air thru 4 stones, Rez temp 67-70, room temp 70-71 with A/C, RH 38-42% under a 400 hps

Imo yes. It usually starts at the tip of the leaves and works down.

What medium are you using? But as I see it now I would flush it out. fresh ph'd water and a flushing solution like clearex or florikleen. Since there is already 250ppms of junk in the water I would cut back on the nutes a little.

honestly it looks more like salt burn/build up.. when the leaves curl that heavy its not just nutrient burn... but obviously the overfeeding would cause the salt build up...

how often are you feeding your plants just plain clean water?