Xxxx are one cause of water pollution that is not well known. There are more and more xxxxx around the world. This is due to the fact that the wild fish in the oceans are disappearing because they have been overfished. At the same time, especially in developed countries such as the United States and Europe, the demand for fish is increasing. In the oceans, fish do not cause pollution, but in xxxxx, the situation is different. Many fish are kept in a small amount of water , which means that the water must be changed often. Each time it is changed, the dirty water is goes directly into a river or ocean. This is very bay for the plants, animals, and fish living naturally in that river or ocean