So, red stems are good? Cuz mine are bitchin red!

I'm having a similar problem - my pH is 6.9 in soil, my temps are good, and I'm being careful not to feed or water too much, yet I'm loosing fans and others to a creeping yellow with brown spots.

I tried a slight boost of bloom mix with the last watering but it didn't do anything. Could it be a fungus, and if so, can that indeed be spread by a kind of gnat?

As for giving it some Cal Mag, would a shot of garden lime and water help out?