George;

Look at the chapter in Eric Borneman's book for a lot of good info about introducing new corals to your tank. Most people do not quarantine animals like shrimp, snails, and crabs. Quarantining corals is probably a good idea, because some corals do conain infectious diseases, but people typically don't do it because your quarantine tank must contain an environment suitable for them, which means strong light and water wovement, and excellent water quality.