It is stated in the passage that …………

When people think about sources of water pollution, they do not usually think of aquariums. However, as the demand grows for fish to eat, the number of aquariums is increasing. In some areas, they are beginning to create environmental problems. In fact, when fish are in their natural environment, they do not pollute. But in aquariums, the situation is not natural. There are usually lots of fish in very little water. This means that the water must be changed very often. And each time it is changed, the dirty water must be thrown away. It is usually poured directly from the aquariums into a river or the ocean. The chemical balance of the river or coastline is changed by this dirty water. And the plants and animals living there may suffer.