Got small problem with plants

I have 2 plants around 16inchs and the bottom leaves are turning yellow, It seem's to be spreading up the plant now, I have flushed one of them and left the other one as it is, i added new nutrients with the one i flushed, Nutes are called Ionic, can any one tell me any ideas that might have caused this problem, so i dont make this mistake again???

whats the ph?
what are your ppm's at?

the ph is 6.5
It is getting 3.5 m of each bottle there is 3 bottles of Ionic
at 200 hundred/ml of the solution in each watering when the
pots feel light

what kind of water are you using? medium? are you adding anything else

not a ph problem or it would be all over, the answer is in your wuestion. Odds are bottom leaves are getting light due to being 16inches

can you take some pictures so we can see and get you some better answers please