Hi,
the problem seems to be diet rather than numbers. I love Anthias, but have never kept one for this reason. They are planktivores and are extremely prolific on some reefs. Due to this the seem to be a little hard to feed, as it is very hard to get a constant supply of plankton into the tank. They can be sometimes hard to feed. I've watched a number of them over periods of time, especially at LFS, because I have been very interested in purchasing one. Due to their schooling nature, perhaps they feel less stressed in groups which is why they do better. However, like all fish, I'm sure there is much variance so IMHO the best thing to do is just to monitor one fish closely for a while and see what happens. I am still doing this and may very well get one shortly.
Tom