tips of leaves browning

Currently in the third week of growing, White Widow (feminized seed). The leaves are browning (starting at the tips) and we can't figure out why.

My Grow Two thousand watt hps 16 plants under each light 16 og kush plants and 16 grape ape plants 3 gallon pots Black gold potting soil 2 weeks in flowering Temp in room gets no higher than 77 degrees Using flora nova bloom 1/4 teaspoon per gallon Feeding them every 3rd watering So for the...