White widdow exstr auto flower(could really do with some advice)

Hi have 2×600 watt hps at 24 inches above the plants....the ph is at 6.5 to 7....i water every. 2 to 3 days i use nutes every other water....

But the bottom leafs are turning yellows the really big fan leafs....


My babys are about 2 weeks i. To flowering can any one give me an idea on how to fix this.....thes are the photos onley the 2 big ones have the problem......

Always the first to go... My green crack are 20 days into flower and lost a few bottem leaves... Ppm around 600.