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Monterey Bay Aquarium

http://www.aquariumsite.org - The largest link directory for aquarium and tropical fish information.

About.com has more than just fish but it has a great section a saltwater. 

Exotic Tropicals - Nice encyclopedia

Wetwebmedia  (Robert Fenner author of - The conscientious marine aquarist -) 

Coral Reef Network

Australian Institute of Marine Science