First Grow - Yellow/White spots

Hey Guys,

First attempt here and I'm having a bit of a problem. I've got a friend who's been doing it for years, and he's helped me setup a hydro table.

It's watering the table every 20 minutes for 7 minutes at a time which is how long it takes to flood the table and then soak for a minute. These are brand new clones that I've got setup in Rockwool. I've got 2 600W lights setup running 18 hours a day and they are quite a bit above the plants so I don't think they're burning them. They are about 2 feet above the plants currently. The PH in the water is 5.9.

Does anyone have any idea what this discolouration is? Is it too hot and burning the plants? Not enough nutrients? Watering too much or not enough? Or is it mildew?

I've had another 8 plants that were started on this same table that seem to be doing excellent, I've had to move them to a new table now to make room for these ones.

This isn't happening on most of the plants, only a small portion. Is it just natural that a few might not succeed or is there something bigger I should be worried about here?

Any help would be greatly appreciated.

Thanks

try to get you ph at like 6.5, I always keep my ph at 6.5.....and now mine look great
and I dont think you should be using nutrients this early, how old is it?