You are in early stage of flowering so you wanna get something that is low Nitrogen, but high in P/K and increase the dosage per gallon over the next few weeks as they enter aggressive flowering.

The slight boost of Nitrogen will help green up some of those fan leaves but yellowing and leaf loss is completely normal throughout flowering, as the plant runs out of nutrients and takes from older less used parts.

I always supplement with some "black strap" molasses, which often times will be an additive for nutrients, but I like to put a little bit more towards the end. What that does is cause the buds to have a bigger yield- the theory is that the extra sugar helps to feed micro-organisms in the soil and that gives a boost to the plants.

But as always keep it simple and go slow with fertilizers.