coral reefs only occur at great depths, most hobbyists are largely confined to fish, sea anemones, crustaceans, echinoderms, mollusks and Feather duster worms. A few corals can be found at low depths. Since there are very few commercially available coldwater fish and invertebrates, hobbyists usually have to physically acquire specimens, although recently more specimens have become commercially available from the west coast of the United States as well as Japan, Australia, and the UK. The most common methods of acquisition are by trolling or seining, and experienced hobbyists use the movement of tides and searching methods to find certain species. Unlike commercially available tropical fish, whose behavior patterns and tank compatibilities have been well documented in the last five or six decades, coldwater fish have been kept in public and private aquaria for over two centuries and much ichthyological knowledge has been gathered in order to maintain them. Many temperate fish have specific local diet requirements, while others, will eat just about any crustacean or frozen foods. Some fish should not be kept with fish small enough to fit into its mouth, crabs or mollusks. Similarly some crabs can not be kept with some mollusks, while other fish, crabs, mollusks and echinoderms may be compatible with each other. It takes experience before one can successfully gauge the compatibility of the fish and invertebrates in one's area. Due to it being such a localized hobby in the United States, not many people go the route of local tanks which are far more popular in Europe. Live rock Live rock is rock that has been in the ocean, composed of limestone and decomposing coral skeleton, usually around a coral reef such as those around Fiji, and is usually covered with beneficial algae, coralline and tiny invertebrates and bacteria that are desirable in the aquarium. Some examples of the microfauna commonly found on live rock are crabs, snails, feather dusters, brittle stars, starfish, limpets, abalones, and an occasional sea urchin, sea anemone, coral, and sea sponge. Also, if the aquarist is unlucky, a mantis shrimp. Bristleworms are also common, most of which, while unattractive, are not harmful and are useful scavengers; some species can be pests, however. The addition of live rock is one of the best ways to ensure a healthy aquarium, as the rock provides a buffer to maintain high pH (8.0-8.3), alkalinity, and acid-neutralizing capacity. Alkalinity is often known by