white tips

These seeds sprouted I'm February I had no real enclosed space but they did great and imo still are doing great. All that being true I noticed just now some white tips on one of my plants but I'm not sure what is causing it. Any help appreciated. Soil: peat moss, perlite, charlies coop poop...

Second grow. Critical kush. 5 weeks veg. In 3rd day 12 12. Medium premier tech bilogical potting mix. Nutes. None in veg. 20 percent pnk nute with 5 percent nitrogen now. Light 600 watt castinoo led. Height of light 24 inches. Yes inches. Lol darn centimeters! These plants are growing well but...