There are both philosophical and scientific aspects to this interesting question! People keep fish inside of containers for a variety of reasons. Some people do it to keep fish as pets, and others to farm fish for food. Either way, fish need to be kept in water.

Fish naturally live in water and have evolved to obtain their necessary nutrients and chemicals in an aquatic environment. Even though they need oxygen, fish cannot breathe air directly, as we and many other animals do. Instead, they gulp oxygenated water and pump it through their gills. The oxygen in the water diffuses into the deoxygenated blood in the gills. The newly oxygenated blood then gets circulated to the rest of the fish's body, and the deoxygenated water that's left over filters through the gills.

People like to look at the fish, so they put the fish in fish bowls or fish tanks with water, or maybe salt water, if the fish are salt water fish.

Fish are aquatic animals. They live in water. The water that they live in is liquid. However, it's liquid water, not some other liquid substance, that fish live in. If you were to put a fish into, say, gasoline, which is also liquid, the fish would die very quickly.

Great question. Humans, like me and you, are mammals that need to breathe air. We breathe air because it has oxygen in it, which our body needs to function every day. Fish need oxygen too, but they get it through breathing water. We put fish in containers with liquid water because that is their habitat and has been for hundreds of millions of years!

Fish evolved so that they have things called gills instead of lungs. If a fish were taken out of water, the gills wouldn’t be able to get the oxygen and the fish would not survive. It would be like if me or you were put underwater… our lungs would not be able to get the oxygen and we would not survive!