Yellow tips

Ok I have 2 plants about six weeks into veg. 1 NLxBig bud in 1x1in rockwool cube in 5gal pot of hydroton and 1 Afghan Kush in 1x1in rockwool in 3 gal pot of hydroton. I feed every day or every other day depending on how dry the rockwool and hydroton dry out with General Hydroponics aggressive vegetative growth ( I'm assuming 3/4 strength) under 2 4ft flourescents. Humidity stays around 40% and temp is around 75'. The problem is that my lower leaves keep yellowing at the tip and die. The plants still seem strong and growing because the stem at the base of the plant is still getting thick . Want to start cloning but don't have any stems that look long enough. Any advice or info would be greatly appreciated... PLEASE HELP!!!!!

They're hungry.

So I'm not feeding them enough???

Any other suggestions??

Feed them a bit more. Make sure your ph is on so u know they are getting it. See the way they are yellowing from bottom up, tip to base (on the leaf) Thats the order the plant will begin to eat itself in if there isnt enough food,....

Thanx 4 the info...