Euphelia are interesting in that hammer nad frogspawn can touch each other, but torch 9in the same family) can not touch either of the others without them stinging each other. Most corals should be kept ftom physicaly touching each other as one will usually kill the other. As far as being completely incompatible though, nothing comes immediately to mind. I know leather corals are pretty noxious and will emit chemicals into the water that stunt the growth of other corals (notably SPS), but there are plenty of tanks out there with leathers and
acros in them. They seem to be fine as long as water changes are kept up, and I know a lot of them run carbon to combat this chemical warfare.
