your plants are healthy enough to recover from this.
first step, check your soil ph levels make sure its around or just below 7 (ph effects the plants ability to take in certain nutes and is often misdiagnosed as nute dissorder) also check the ph of your nutrient solution, and the ph of the water runoff after it comes out of the pot again you want these to be at or just below 7
check the water you use, make sure there arent high levels of contaminates like chlorine, flouride, sulfer, salt, and other things. this can easily cause symptoms like yours after a time as the contaminates build up in the soil and cause nute lock.
if ph is good, it could easily be nutrient lock from overfertilization flush your plant with light nutrient solution (just barely any nutes) to get rid of excess nutrients
if all of that is not the problem, then it could be a mobile nutrient difficency. mobile nutrients are nutrients that move to where they are needed in the plant, mainly to new growing shoots, if they are deficient then the older lower fan leaves are the first to go. the big mobile nutrient is one the plant uses most often Nitrogen. others include phosphorus, potassium, magnesium, zinc, and a few trace elements, which nutrient is deficient can be determined with close inspection of the yellowing leaf and the traits it shows.
all that said, in late fall when plants are flowering it is normal for the fan leaves to yellow and fall. think about the trees in autum when their leaves change color and fall off. cannabis reacts to autum in a similar way.
good luck