It is a nutrients deficiency , yellowing is usually caused by a lack of Nitrogen. Your actual nutrients amount is poor/weak so you should increase the dosage right now and if you wanna stabilized your plants then use some Bio grow to bring some more Nitrogen and stop teh yellowing ( the affected leaves won't recover ) Also why are you cutting the edges of the leaves ? if it's because they are burning or something like that thn stop doing it becase i could be a sign of another problme thay we won't be abble to diagnosis if we can't see it