either Baensch's Atlas or Exotic Tropical Fishes by TFH. They are informative and have lots of identifying pics with information about the species. The drawback is they don't cover every species, only the more common ones. If you go for Baensch's Atlas they do 3 or 4 volumes in fresh and a similar number in salt water species. The first volume in each contains a lot of the more common species plus general aquarium set ups and filtration, etc. The next volumes have more unusual species and new species.