WOW, nobody has a clue huh??? You have the beginnings of a toxicity, you need to flush with a mild nute mix (1/2 strength of what's recommended) and add Cal-Mag to your feeding schedule, it's the Mag def giving the "burnt rusty ends" look....AND, most definitely do not water in soil with R/O water, use tap water left out for 24 hours to evaporate the chlorine it has chelated minerals your plant needs....good luck