The tips turn brown because of nutrient burn. You use too much nutritions. This is just te very beginning from a nutrient burn so good that you noticed it early!
To solve this problem just cut back the nutrients to 75% from your last given dose. The tips that are brown will stay brown and won't recover. But when you cut back to 3/4 of the previous given dose the rest of the plant will stay green and the plant will not be harmed further by overfertilizing.