首字母when people think about sources of water p（1）,they do not usually think of fish farms.However,as the demand grows for fish to eat,the n（2） of fish farms is increasing.In some areas,they are beginning to have environmental problems.I（3） fact,when fish are in their natural environment,they do not pollute.But in fish farms,the environment is n（4） natural.There are usually l（5） of fish in very little water.This means that the water must be changed very

首字母when people think about sources of water p（1）,they do not usually think of fish farms.However,as the demand grows for fish to eat,the n（2） of fish farms is increasing.In some areas,they are beginning to have environmental problems.I（3） fact,when fish are in their natural environment,they do not pollute.But in fish farms,the environment is n（4） natural.
There are usually l（5） of fish in very little water.This means that the water must be changed very o（6）.And each time it is changed,the used water must be thrown away.It is usually thrown directly f（7） the fish farms into a river or the ocean.And that changes the chemicals of the river or coastline,so the plants and animals living there may suffer.