How many times have you tried Acros? What was the source of them and in what form? LFS? Private person? Fresh Cut? Healed? How did you put them in the tank, i.e. left on frag mount or removed and glued in place? Your parameters seem good and really, adjusting any of them up or down just a tad is not going to do anything. The important thing is to keep them stable at a given level. Were your parameters stable when you had problems with them? Even a single instance of fluctuation can RTN a piece, especially if they are new to your system. Lastly, all Acroporas are not equal. Some can be tough as nails while others seem to RTN if you look at them funny while passing by the tank.