I’m not a fan of most Anthias but either way your tank’s too small for a trio, or even one. If you really want an Anthias, maybe a Sunburst / Fathead but they’re not your typical Anthias as in they don’t school. If your tank was 120+, as in at least a standard 4 x 2 x 2, maybe you could do a trio of 1M / 2F and I think that’s about the limit for most tanks. You don’t want so many specimens that a single male can’t control them with his amount of hormones, you don’t want a second male to emerge.
It’s tricky to establish a true school or shoal of most fish in captivity. In the wild, many species only group as juveniles, or are found in groups as a survival mechanism. They don’t like being around one another and upon closer inspection of individual specimens you will see wounds and torn fins and scales from infighting.
Even in a tank as big as Andrew Sandler’s, who keeps a large group of mixed Zebrasoma (Yellow and Purple, with a few aberrant Scopas, Sailfins and Black Tangs mixed in), many of the Yellows in his videos, you will see that they have incomplete fins from fighting with others of their kind.
A lot of Tangs swim in groups, but what you see as shoals are really just pairs / trios swimming together for survival and foraging.
Butterflies exhibit a range of social structures. Many exclusively form male / female pairs, others are more like the Tangs that form shoals in that a shoal is an aggregation of smaller units that stick together for safety and feeding.
Take Bangaii and Pajama Cardinals, many people want them because they have the reputation of being a schooling fish, but when they get older and are of breeding age, they exclusively mate in pairs. Odd ones out are at best, left alone, at worst, killed off.
And Chromis, it’s luck. “They will decide their own numbers, so always buy more than the number you want” seems to not be an uncommon sentiment.
I don’t consider most Anthias, with a few exceptions, to be individually attractive fish. For me, the criteria is that if I keep a fish singly, it has to be a species that’s individually appealing. Or, it should be a species that I have a reasonable chance of having in a pair or trio (while looking decent individually) if that means getting the most out of them.
Though not suitable for a 40 by any means, it seems a good number of people have success keeping the Hemitaurichthys (Pyramid Butterflies) in trios long term.
