Dying and yellowing bottom fan leaves

Of my four plants the two larger ones started of growing well but now the bottom fans leaves had yellow tips and began to curl a little have turned completely yellow and i do not know what could be causing this.....

I have good ventilation and an oscilating fan, the temp never gets above 78 and i ph balance the water around 6.5. finally i just witched over to flowering and i am have been watering them with flora nova bloom added into the water which i balance after adding the fertilizer.......

did you repot them before you switched if you did new soil has enough in it for a while could be nutes burn

no i did not repot them