The following are websites that I personally thought contained useful information in our aquarium hobby. Although I have found the website informative and entertaining, I hold no responsibility over the information they provide. When in doubt, you should always seek a licensed veterinarian.
First Tank Guide – I referred to this site a lot when I first returned to the hobby six years ago. The author Keith Seyffarth does a great job laying out information and explaining concepts to the beginner aquarist.
About Fish Online – Information on freshwater and marine fish keeping, aquarium maintenance, species profiles with photos, and fish health.
Aquarium Links – An index of aquarium related websites.