Another lesser known "fact" (I have owned several over the years) is that they seem to prefer slightly saltier waters. If kept in a FOWLR, you'll want salinity over 1.025 IME. I've always been able to keep them most successfully in the 1.027 area, they seem to prefer it. Other inhabitants don't seem to mind it, either.
Several "OG's" in this hobby that have kept achilles and sold many over the years have told me of this trick as well. The flow is equally if not more critical.