Your nute temp is too high and expelling the oxygen. Put lots of air-stones in your res and get a fan blowing over the water. That'll get the temp down some. Be sure to cut all the brown roots off, they're already dead. What i do, sense i check my girls everyday anyway, is to keep an extra gallon of ferts in the frig. Take a gallon out of you res and replace it with the cold gallon about 30 minutes before. Place the gallon you took out in the frig and just rotate a gallon every day. Your nutes needs to be around 55degs or you have to get oxygen into the water.
Add the stuff LOC mentioned if you agree with it or just change your nutes.