Fish are a source of protein in many diets, humans consume a lot of fish for this reason. Fish farms are used to supply fish because: the fish's food safety standard is monitored; fresh water fish are declining; the deman for fish is increasing.
Fresh water fish may have come into contact with unclean water; containing sewage, waste, chemicals. In fish farming the water is filtered to make sure the fish don't come into contact with anything it would be unsafe to digest. Also if the water is cleaned regularly the spread of disease is minimised and the oxygen levels are high enough to maintain the respiration of the fish.
Intraspecific predation is the fish being bred eating each other, this can be stopped by: separating fish of different ages; separating fish of different genders; feeding fish regularly; giving fish adequate room.
Interspecific predation is the farmed fish being prayed on by other species, this can be prevented by: fencing the area the fish are in; putting nets around the area the fish are in; keeping the fish in inside tanks.
To minimise spread of disease, the water the fish are in should be changed regularly and their surroundings sterilised often. Also if small amounts of fish are kept together then disease can only contaminate a few fish.
Waste can be removed by changing the water in a tank, or changing the nets and location of fish kept outside.
Fish need to be fed often in small amounts, this is so they don't starve but they wont be able to over eat. It is important to feed fish food with nutrients in for growth.
Selective breeding can ensure that farmers produce fish with desired characteristics by letting only the fish with the right characteristics breed and pass on the gene.
A useful source is: http://www.scribd.com/doc/79750837/Fish-Farming