ReefCentral One of the busiest boards with some knowledgeable people. Eric Borneman has his own coral forum here.

UltimateReef the most active of the UK boards

Reefs.Org - US board. Lots of good articles in the library section.

Aqualink - lively US board

IKS - computer control for aquariums

TMC - main livestock importers in UK and makers of hydrometers

Durso Overflow - quieten that overflow

Stockman Mod - mod for smaller overflows

Aquarium Frontiers - archive of some good articles

Salwater mining - regular trawl of the net for marine info

Unit Conversion calculator - handy for converting litres to gallons etc

Hyposalinity - the best way to treat fish for parasite infections like ich

Craig Bingman's site - lots of good articles that explain water chemistry

Andy Hipkiss site - SPS corals and some good chemistry explanations plus a handy alkalinity to calcium calculator

The Reef Aquarium Vol 1 & 2, J Sprung and C Delbeek - bit old now but a great intro to reefkeeping and lots of common sense advice.

The Modern Coral Reef Aquarium Vols 1, 2 & 3, - another good intro

Aquarium Corals, Eric Borneman - one of the best coral books.

Marine Fishes, Scott Michael - the best quick guide to fish compatibility and suitable tank size.