Marine aquariums are for fish that require a saltwater environment in the wild. Saltwater has higher degrees of dissolved salts than freshwater bodies of water; examples of saltwater are oceans, bays, estuaries and sounds. The water temperatures for marine aquariums should average around 55 degrees, similar to waters in the Pacific Northwest, Antarctica and the North Sea. The list of saltwater fish for marine aquariums includes the Catalina goby, truncate coralfish, blue devil, grunt sculpin and boxfishes. Aquarium owners with saltwater fish and shellfish should buy two tanks so the fish will not prey on the anemone.
In the wild, freshwater fish are species that live in rivers, lakes and marshes. Some coldwater fish for a freshwater aquarium are gars, snakeheads, pygmy sunfish, barbs and swordtails. When buying a tank for a freshwater aquarium, several steps must be taken to insure a healthy life for the aquarium's fish. Constructing an aquarium and making sure it has an efficient biological filter takes approximately six to eight weeks. Also, freshwater aquarium owners must maintain the nitrate and ammonia nitrate levels at zero.
Man-made ponds are for owners of larger fish that are too big for aquariums. These small bodies of water should be at least 3 or 4 feet deep to ensure certain species survive. The temperatures for fish in ponds should range from the low to high 60s. Ponds accommodate large freshwater fish such as koi and carp, which grow up to 2 or 3 feet in length. Another popular fish for ponds are goldfish; this fish reaches lengths of .5 inches to 1 foot long. Other coldwater pond fish are orfe, catfish, sturgeon, tench and rudd fish.
Marine and freshwater shellfish may live in a coldwater aquarium setting. Shellfish in marine aquariums are primarily echinoderms, which are invertebrates with spiny skin and radial symmetry -- which means they do not have left or right halves, only top and bottom. Starfish, anemones, sea urchins and sea cucumbers are echinoderm shellfish found in marine aquariums. Some crustaceans, such as crayfish, some species of crabs and shrimp, must live in freshwater aquariums, while marine aquariums are suitable for lobsters and crabs. Freshwater aquariums also host mollusk species, including oysters, snails, mussels and clams. Buying a separate aquarium for shellfish prevents fish from eating the shellfish.