Newbee Flood and Drain Questions

Hello I am into my first flood and drain grow and I have some nubee Questions
First I am using 4x4 inch rockwool cubes
and i am wondering how high should the water get on the cubes when the pump is on and how long should the water be kept at that level befor shutting the pump off and allowing the water to drain backshould it be a quick in and out or keep it in for a certain period of time?

I keep the solution at a steady ph of 5.8-6.0
my ppm is at 850
Iam using a 600watt hps at 19\5 cycle approx 25" away from the plant tops?
should the rockwool be soaking wet or more of a damp ?
the girls have been growing for about 5 weeks and are showing a very small amount of new growth with the lower leaves showing yellow and all the leaves are dropy
Iwater 3 tmes a day and change the soluion every 2 weeks
I am afraid I may loose the girls soon if i can't figue out the problem
please help

thanks

How old are they ??? Are they clones or are they rooted?? If they are rooted.... do u have them in containers?? What medium ?

They are rooted and are about 6weeks old
I have them in a 2'by 4' tray in 4'cubes covered with expnded clay pellets

Your water should just reach the bottom of the rockwool cubes. The cubes hold water for a long time, and the disolved oxygen will disapate before the water is gone.

Yellowing leaves and slow growth will both result from the situation I describrd above.

I run the pump 3 times per day for 15 minutes at a time, but only when the lights are on - once just after the lights come on, once midway thru the light cycle, and once just before lights out.