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Some 600,000 Americans keep saltwater and marine aquariums, because of their love and fascination for the reefs and their spectacular fish. More than 20 million ornamental fish from 1,400 different species are traded worldwide each year, with many of them captured from the wild through the use of fish-stunning cyanide. The side result: More than half of the fish in the vicinity are killed, along with many fragile marine species such as coral polyps.

Still, you can create an eco-friendly saltwater aquarium, says David Helvarg, founder of Blue Frontier, and author of 50 Ways to Save the Ocean (2006, New World Library). Here are some guidelines you can follow, as excerpted from Helvarg's smart and entertaining book.1. Consider owning a freshwater aquarium. It is easy to set up and maintain one, and more than 90 percent of freshwater fish sold as pets are captive-raised. Some African cichlids rival the color and beauty of their marine cousins and breed readily in home aquariums.

2. If you own a marine aquarium, purchase tropical fish that have been reared in captivity. Aquacultured species you can now buy include clown fish, dottybacks, cardinal fish, gobies, batfish, sea horses, and several interesting invertebrates such as peppermint shrimp and snails. The group Reef Protection International produces a pocket guide to help you select sustainable aquarium fish.

3. Join a hobby club or contact more experienced aquariums to learn how to captive-breed and trade your marine plants and animals. Several excellent online resources are available, such as Reef Central, where experts answer questions for new hobbyists.

4. Patronize aquarium shops that are environmentally aware. Hobby groups such as the Marine Aquarist Society and advocacy groups such as Reef Protection International can help you locate them.

5. Ask about the origin of all fish you purchase. If they are not captive-bred by a company such as ORA, Inc., make sure they have MAC certification, which assures they were sustainably captured.

6. Never dump anything from your tank into a storm drain, lake, river, bay, or other body of water, because you could be introducing harmful non-native species or microbes. ::50 Ways to Save the Ocean

Difficulty level: Moderate