yellow tips of leaves

Nitrogen is a mobile nutrient, a nitrogen deficiency will start showing in the lower older leaves first and work its way up the plant. Yellowing tips at the top can be caused by a number of things, but it seems you have some narrow leaves which can be a sign of excess K. Excess P and K are common due to the bloom nutrients in use, while the plant does require more P and K in flower, you can still have too much. Check your feeding levels, and make sure you are allowing sufficient dry time between waterings. Happy growing.