What about water flow inside the tank?
81 is a bit high, though that's not likely to cause the problem. However, it could be adding to any stressors present. How are you gauging the temperature? Glass thermometer, stick on, digital?
You could stick a small fan blowing across the surface of the water to bring the temp down.
Salinity is good. Are you topping off evaporated water to maintain that salinity?