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Plants and animals that live in water make up the majority of biomass. They have so much more space in which to live. Life undoubtedly originated in the water, and many life-forms have never left it. Living in a dense fluid like water provides a lot of support for organisms, and the free-swimming forms have three-dimensional mobility. And, of course, they never have to worry about where their next drink is coming from.

For many of us a goldfish bowl was our introduction to aquatic animals. The beautiful orange fish are hardy and forgiving in terms of their living requirements. They are good classroom animals, requiring a minimum amount of care to keep them in good shape.

Goldfish weren't always gold; their wild kin are dark gray and olive-green. Goldfish are related to carp, so they have a rather unglamorous lifestyle, spending their time foraging in murky water for a variety of foods, including plants, insects, snails, and the eggs of other fish. The lovely colors and sometimes bizarre shapes of goldfish are the products of selective breeding conducted in China and Japan, where they are native.

A bucket, a bowl, a plastic bag, or a two-story-tall glass box can be an aquarium. A few of the millions of plant and animal species make excellent classroom aquarium organisms. But before we look at the organisms, it is important to think about the factors that make up their environment.

Water. Chlorine in regular tap water can be lethal to both goldfish and guppies. There are two ways to dechlorinate water. The first is to age the tap water by letting it sit in an open container for at least 24 hours. Chlorine dissolved in the water escapes into the air. Or add dechlorinating chemicals (included in the kit) to tap water. In some water systems, chloramine, a newer additive, is used in place of chlorine, and it will not leave the water when exposed to air. You must use water conditioners that specifically say they remove chloramine. It might be beneficial to ask at your local pet store or aquarium store just what is recommended in your area to make the water safe for fish. Set aside a pitcher of water to age, so that you will have it ready to maintain the water level of the aquarium. Keep your aquarium covered to reduce evaporation and to keep dust out and fish in.

The water in an aquarium will be fine for extended periods of time. Aged or treated tap water should be added to maintain the proper level, and about once a month half of the water should be removed and fresh water added in order to reduce the concentration of nitrogen-containing chemicals, the excretory products of the animals.

If a fish dies, or if too much food is put into your aquarium, change the water immediately. If you don't, bacteria will proliferate, taking advantage of the bounty of food. The result will be a putrid smell and danger for your fish and other aquarium animals. The bacteria will quickly deplete the oxygen supply in the aquarium, and the animals will suffocate.

Temperature. Unless you get fish that are specifically identified as tropical fish, it will not be necessary to obtain an aquarium heater. For the classroom it is best to stick to temperate-water organisms—they are so much easier to maintain. In fact, it is more important to keep your aquarium from getting too warm. Warm water holds less oxygen in solution, so aquatic organisms are more comfortable in cooler water. Keep your aquarium away from direct sun except for specific purposes.

Aquarium care. Goldfish don't place many demands on the aquarist. They need unpolluted water, but it is not necessary to provide extra oxygen with an air pump. You may experience some mortality when you first introduce new fish into your aquarium, but this is often due to transportation stress. As long as the fish are not crowded, they will be able to get enough oxygen just from what is dissolved at the surface of the water. To avoid crowding, do not exceed one feeder-size (3 cm) goldfish per liter of water for any extended length of time. You should be able to put six to eight guppies in a basin aquarium.

Food. Goldfish will eat a wide variety of foods, but the most convenient is a commercial flake food. This kind of food floats, and the fish will quickly learn to come to the surface to eat. The most important thing about feeding is not to overfeed! Feed your fish once a day as much food as they will consume in 3–5 minutes. Too much food left in the aquarium will foul the water. Fish-feeding cakes—compressed food that disintegrates slowly—are available at pet and aquarium stores if you need to leave the fish unattended for more than 3 days.

Feed the fish when students can observe the feeding behavior. Goldfish like to eat insect larvae, worms, aquatic plants, and snail eggs (all of which they eat in the wild) as well as commercial food. Guppies also eat commercial fish food, as well as finely chopped fish, tubifex worms, earthworms, and Daphnia.

Reproduction. Goldfish are very prolific in nature and in special breeding ponds, but don't expect any offspring in the classroom. They need lots of plants and other cover to propagate successfully. If they did lay eggs in the aquarium, they would doubtless eat all of their own eggs during their incessant foraging for food morsels. Goldfish can grow to be 40 cm (16") long and may live more than 10 years. Be prepared for an extended stay when goldfish move into your room.

Guppies are small fish that bear live young. The feeder-guppy females are larger and usually a uniform beige or silver gray. Their abdomens become quite large when they are gravid (carrying young). The males are smaller and have longer, flowing tails. Males are the ones with spots of multiple colors. Fancy guppies that have been bred for showy colors can be dazzling.

Guppies are quite prolific and will probably give birth during their stay in your classroom. In fact, you may observe the arrival of baby guppies a day or two after the adults are put in their basin aquarium. The stress of transportation may induce a gravid female to release the babies. Adult guppies will eat the young, so you should supply the aquarium with plenty of Elodea in which the babies can hide, or move the adults to a separate tank. Students will enjoy watching the baby guppies grow.

What to do when the fish arrive. Float the unopened bag in aquarium of dechlorinated or spring water for about 15 minutes to equalize the temperature. When temperatures are equal, pour contents of bag through a dip net into another container and transfer fish from net to the aquarium. Discard shipping water. DO NOT USE CHLORINATED TAP WATER!!!

Maintain aquarium at room temperature out of direct sunlight, adding and/or changing water with treated water as necessary to reduce the concentration of nitrogen-containing chemicals naturally occurring in the water.

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