Higher nutrients do not cause “brown” acros in my experience. Higher nutrients have caused darker/deeper colors but not “brown”. If your corals are turning brown and “dried up” with thin tissue than you they are stressed from something. Starvation from a lack of input versus export or irritation from pests or a contamination is most likely to be the issue
It's been my limited/brief experience as a new acro keeper that some coral frags turn brown upon introduction to my system while other coral frags out of that same system did not.
So it may very well be species sensitivity varies among certain coral when experiencing a rapid change in parameters.
Have recently switched to Acro Power ammino acids and have noticed a transition out of the 'brown" phase after my first dose.
This coral was completely brown but is beginning the transition back to coloring up beginning at the base.
It use to happen to me all the time last year, never occurred to me to check and keep phosphates to where they should be..phosphates high and swings was my issue. Once I got phosphates under control my corals took off and no more browning