Fertilizer
Over-fertilizing is usually a bigger problem than under-fertilizing. Any balanced (20-20-20, 14-12-14, etc.) water soluble fertilizer, applied at 1/8 to 1/4 of a teaspoon per gallon of water can be used. Try to use one that's urea-free. During the winter months, fertilizer may be omitted or reduced, unless the plants are grown under lights.