The American Fisheries Society http://www.fisheries.org/
The Florida Museum of Natural History http://www.flmnh.ufl.edu/
Neodat http://www.neodat.org/ freshwater fishes of the American tropics.
Tree of Life http://phylogeny.arizona.edu/tree/ fascinating site about evolution, biodiversity, etc.
General, commercial
E nature http://www.enature.com/ a commercial site with nice species profiles and other links.
Other animals
Sea turtles http://www.seaturtle.org/
Arthropods http://www.sasionline.org/
Reptiles http://www.embl-heidelberg.de/~uetz/LivingReptiles.html
Freshwater and Marine Aquarium
Tropical Fish Hobbyist
Aquarium Fish
All three are helpful with articles and as sources for various products.
Water Quality
Captive Seawater Fishes, Steven Spotte, 1992
John Wiley and Sons Publishers
ISBN 0-471-54554-6
This is probably more suitable for professionals and advanced hobbyists. Definitely not for beginners.
Marine and Reef and Freshwater Aquariums
The Reef Aquarium:
A Comprehensive Guide to the Identification and Care of Tropical Marine
Invertebrates, Vols. 1 and 2
Julian Sprung and Charles Delbeek, 1997
Ricordea Publishing
ISBN1-883693-13-6
Svein A. Fossa and Alf Jacob Nilsen, 1996
Birgit Schmettkamp Verlag
Bornheim, Germany
These are both excellent books.
Tetra Press Aquariology Series, Four Volumes
The Science of Fish Health Management
ISBN 1-56465-105-3
Fish Breeding and Genetics
ISBN 1-56465-106-3
Fish Anatomy, Physiology and Nutrition
ISBN 1-56464-107-X
Fish Diseases and Water Chemistry
ISBN 1-56464-108-8
A nice set of general reference books, lots of good information.
Fish Disease:
Diagnosis and Treatment
Edward J. Noga, 1995
Mosby Electronic Publishing
ISBN 1-55664-374-8
A good source for veterinarians, professionals and advanced hobbyists.
W. B. Saunders
ISBN 0-7216-2629-7
A comprehensive book for veterinarians and professional managers.
