that is one of the weird things about allelopathy(production of toxic compunds)in corals.Corals that are seemingly healthy for periods of time can all of a sudden recede.Placement in the tank will have little to do with it as the causitive factors are chemicals that are released in the tank.The only thing you can really do is run activated carbon 24/7 as it will help to remove these chemical toxins.I remember Reefluvr having the same problem with a Euphyllia and a Sinularia a while back.