If the tips are curling under you have a magnesium deficiency. This is pretty common if you're growing inside. Calcium and salt buildup from nutes and your water supply cause this problem. To fix it, flush your pots with enough water to completely fill the pots about 5 or 6 times. Just use your tap water, because bottled water is going to cost too much. After you do this, add 1 tablespoon of Epson Salts to one gallon of water. Water each plant (after they dry out from flushing them - about 2 - 3 days), with 1/2 a gallon of the Epson salt water. Do this for about 2 or 3 times, then hit them with about 1 tablespoon of Big Bloom in a gallon of water, 1 1/2 cups each plant twice a week for a week. Then resume your normal feeding schedule. NEVER feed according to the bullshit schedules on the product label! Start with 2 tablespoons of Big Bloom in a gallon of water, and give each plant about 1 cup every third watering. Then when they're 3-4ft tall, add 1 teaspoon of Grow Big, with two tablespoons of Big Bloom every 3rd or 4th feeding. If the tips start to curl, back off and just use water. When blooming, use 2 tablespoons of Big Bloom, and 1 teaspoon of Tiger Bloom every third water. Smaller amounts of nutes will yield much better results. The Fox Farm people just want you to use more of their product. They don't give two shits if you burn your plants!