One of the most common symptoms for Nitrogen toxicity is when the leaves turn to a darker green, and the tips curl downward.
I had to face that in my previous grow, flushing with pure distilled water (PH 6.8-7.0) worked perfectly for me.
Mentioning, for the next 2 days after the flush, i just got the lights abit faraway to prevent stressing the plant in that poor condition, then got them back to their sweet spot again.