The reason I asked about A. gemmifera and A. humilis is that I believe these are particularly suseptible to RTN, and sometimes it starts with these and then moves onto others. The last RTN episode I had was in a frag tank. I added a VERY small piece of wild collected A. gemmifera (or very similar species) without giving it the usual Lugol's dip. The RTN spread to much of the frag tank, but I used the chloramphenicol treatment, and saved several pieces. This is one scary thing when it happens. FWIW, the temp was not above 80 F (it was spring in New England). A. gemmifera, and I believe also A. humilis are from a very high energy zone of the reefs. They need very strong water movement, and probably do not have very good abilities to 'clean themselves' the way some Acros might.