PLZ help, hydro flood & drain

Hello my dear fellow growers, i am having a problem, so far i been growing in tent but recently converted my loft in to little grow room, i got cooltube with 2x 600w hps but only 1 bulb is running atm, 50% humidity, around 28C, plants are now more than 2 weeks old, at first they were looking very good but stopped growing 10 days ago and i dont know what is wrong, I am using flood and drain system, Canna Aqua A+B, first week plants were given just water, now 50% strenght solution, EC 1.4 ph 6.0, i am hydroponic grower for last 2 years and never had this problem, really dont know what is wrong and what to do, my plants look like they are dying plz help me! thanks

Once again we get pics with HPS on, when will they learn, anyhow for hydro your ph should be 5.8, tho a difference of .2 doesn't seem like much it could, how often do you feed them, that too could be the problem, if you flood them too often, or too long, you could cause root rot.