I have had the same problem and each time Greg asks me about heat. What was happening for me is that the sun hits the back of the tank through a window at the end of the day probably causing a heat spike. I now have a fu in a QT so I think it is heat. I now have a hanging in front of the window to block the sun and dropped the entire system heat again. But if you put him in a QT, lower salinity, lower the temperature and add vit. c it seems to help.