Those are white flies. Common cannabis pests. You can find stray at any store. Spary the leaves like ones a week very carefully. As far as nutrients go. Your plants are quite established and could benefit from fertilizers. Use them lightly and watch pH. I personally suggest organic but i grow outside. If you want chemical fertilizers, i suggest Advanced Nutrients. They won best nutrient line by high times. They have a wide selection from vegetative to flowering formulas to organics and rooting solutions. They are rather hard to find in my area so good luck. Read the labels on the fertilizers and get a high nitrogen for vegetative and a high phosphuros for flowering. Its quite simple.