They have a lot of yellwo tangs in this tank, strange, but the Georgia Aquarium doesn't have a good varitey of coral fish species,they have only one trigger fish species in this tank, most of coral fish species they have are butterfly fishes.
I know the above are old comments but a few specifications: If the photo was taken in early/mid 2009, this tropical reef aquarium alone had 60-80 fish species. Most species from 7 families: Surgeonfish, cardinalfish, wrasses, damselfish, angelfish, anthias and blennies. Species from several other families also in this aquarium but each of these only with <5 species (2 sweetlips species, 4 rabbitfish spp., 1 batfish, 2 dartfish spp, 1 fusilier, 2 dragonet spp., etc).
Two of the groups mentioned in the earlier comment, triggerfish and butterflyfish, are for the most part completely unsuitable for any aquarium with live corals because they will eat them. Additionally, most triggerfish species will eat small fish and can't safely be kept with many of the species in this aquarium. Consequently, this exact aquarium never had and likely never will have a great diversity from these two families. Most butterflyfish species in Georgia Aquarium are kept in the garden eel aquarium (3-5 butterflyfish species have usually been kept there). I only ever remember seeing 2 butterflyfish species in this exact aquarium (pyramid butterflyfish which is one of a few reef-safe butterflyfish, and raccoon butterflyfish). On my visits, the most I remember seeing at Georgia was about 5 triggerfish species but never more than 2 species at any one point in this exact aquarium (species I remember seeing at some point in this exact aquarium are redtoothed triggerfish, one of the very few relatively reef-safe triggerfish, and picasso and rectangular triggerfish that both were relatively safe because of their small size).
In summary, while quite few fish species are visible on the photo (based on a fast check I can see 11 species) and yellow tangs certainly are the most numerous (at one point there were 500+ yellow tangs, more than 4 times as many as the second most numerous species, lyretail anthias), the species diversity in the aquarium is actually very high. On a worldwide basis, there are very few reef aquariums with a comparable species diversity. However, for some of the less numerous species you need to spend some time looking to see them and for some others, especially the blennies, hawkfish and dragonets, you need luck no matter how much time you spend looking. There have also been relatively large variations in species number; at the lowest I suspect about 40 species were in this aquarium and at the highest almost 90.
