I have to 12" air stones in my reservoir which holds 30 gallons, Do you think they are in water to long? Its on a timer, 15 min, but it takes about 5 to fill. I actually picked up one bucket and the roots were already thru the bottom of it. Should I lowere the water level where the fill line is at?
Wish I could give you a confident answer. I'm guessing for some reason that your roots want more oxygen than they're getting. A cheap(?), easy diagnosis is to tweak your res with hydrogen peroxide, say 125 ml of 30% for your thirty gallons of solution. (is 30 gallons the amount of soln, or is it the res' max capacity? Adjust accordingly up to 5 ml/gallon of actual.)
Otherwise I'd look into a way to get a bit of fresh air into those buckets.
But no; i do not think that being under aerated water for 15 minutes should be the problem. DWC keeps the roots immersed, but compensates with aggressive aeration in each bucket. cn
<edit> How high do the buckets fill? Is there an overflow provision? Is nute soln actively overflowing/circulating during those 15 minutes? Do the buckets drain completely? If not, how deep is the residual?