white pebbels? i have a a bucket of hydroton rock that have been sitting ousside in 80 90 degree temp for a month now that are still red. maybe im stupid and you alread adrees this but it sounded to me like salt build up and maybe over fertazling...over ferting can cause narly burns in your plant. i had the same problem in my hydro setup early in vegging my babies. i just flushed the system with 3 days of only water feeding constanly checking ph each time. if you use a liqued ph tester which im sure you do you want the color of the water when you test it to look like a dark yellow somewhat like when you take a piss after holding it in for awhile