Over-fertilization is usually a bigger problem than under-fertilization. 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, fertilization can be omitted or reduced, unless the plants are grown under artificial light.