I am using two 4 by 8 ebb and flow tables. Two of the tables have 2 600 w hps lights covering them and the other two have 2 400 w hps lights covering them. I am using a rockwool based medium. 4 by 4 cubes to be exact. Then they are placed in 3 gallon black pots on top of a 2 " layer of hydro coral rocks. After that I have put more hydro coral rocks on the sides of the pots. Then a final small layer of rocks to barely cover the growing medium. The plants are about a foot and a half tall. Next I have placed 4 plants under each of the lights. My light cyle has just been switched to 12 on 12 off. I was flooding the table three times a day but I started to notice the leaves turning yellow at the bottom and some brownish yellow on the tips. Also my my veins seem to be a dark purplish color. So i have read a bunch of helpfull information on this site. Now I am flooding the tables every two hours for 15 min during the light cycle. During my dark period only one time. My plants dont really seem to be growing like they were before I put them in the tables...I havent been able to top them in 3 days. Can someone please tell me what my problem is as I dont want to lose my crop?

Little more info would help......

What PH and EC are you running at?
Whats the temp in your room?

ya, impossible to tell without proper ph/ ec and temp readings, but it sounds to me like your plants are smallish and being fed too heavily, possibly with the addition of high heat compounding the issue... with that much light you better have a beefy exhaust. tell us more and we can help you out.

GNF

as for these two statements....
1. the dark purplish colour on then stems, could be caused from cold temps...what are they at lights off?

2. ive read quite a few threads on here wherealot of people would say dont ever flood in the dark periods...but its not a complete no no....dont know whehter it'll actually damage them in any way. ?
let some more exp.ppl try help more...
just my 2ps worth ....gl