Top leaves turn dark and curl - What can I do?

That is due to temperatures dropping to sub 68F or genetics. Fall/autumn colors just as you see in tres outside. So, this wasn't caused by nutrient issues. If you had any other symptoms do consider those, but otherwise resume normal fertigation before it runs out of building blocks for growth. no worries, i've seen significantly more decimated canopies on plants that continue to develop and finish their buds. Any negative impact is probably imperceivable if you get it back on track.