Many of the disorders and diseases that are known to occur in fish are the result of stress, poor water quality, overcrowding, and failure to quarantine any new or sick fish to avoid spread of disease. These factors can all be minimized by appropriate care and good hygiene. Infections caused by bacteria, protozoa, viruses, fungi, or parasites may also occur.

Gas bubble disease
Fish are cold-blooded, which means their body temperature is close to and fluctuates with the temperature of their environment. In addition, all their bodily processes are greatly influenced by the water temperature. Water that is very cold or that has been under pressure can become supersaturated with dissolved gases. If the temperature rises or the pressure drops suddenly, these gases may expand rapidly. If fish have already been exposed to this supersaturated water, the gases they absorbed while breathing may also expand rapidly, releasing gas into the bloodstream. This is called gas bubble disease, and the small bubbles created can result in much tissue damage and death. Gas bubble disease in pond fish can be caused by owners filling an outdoor pond with well water using a hose. If the hose is submerged, gas in the incoming water will stay dissolved in the water and can cause problems. This is especially important if the water source is a deep well. To prevent this, the inflowing water can be sprayed as it hits the tank or pool.

Too much ammonia
Excess ammonia in a system is very harmful to fish. High levels of ammonia in an aquarium can be caused by several factors. Two syndromes that are characterized by very high levels of ammonia are well described. The first is called new tank syndrome and is a simple accumulation of ammonia that occurs when a new tank is stocked with fish before the biological filter is fully functional. In this case, the system will be recently set up, usually within the past 1 to 3 weeks. Ammonia accumulates because there are not enough bacteria in the biofilter to metabolize it. The situation can be managed by frequent (sometimes daily) water tests. When total ammonia levels are high (2 milligrams per liter or higher), at least 50% of the water in the aquarium should be changed. This intense management should only be necessary for 1 to 2 weeks, unless there are other unidentified problems with the system. Water quality monitoring will show a decline in ammonia, followed by an increase in nitrite as the bacterial colonies grow. The process is complete when nitrite levels also fall to normal.
The second type of ammonia problem is called old tank syndrome. While it is also characterized by high ammonia concentrations, it is completely different. This problem is caused by a sudden and drastic drop in pH, often below 6.0, which kills bacteria in the biofilter. The loss of bacteria results in the high concentrations of ammonia, which are the hallmark of this problem. Simple water changes are not recommended, however, because an increase in pH may cause the ammonia present to become toxic, killing the fish.
Old tank syndrome is caused by a loss of buffering capacity, which allows the pH change to occur. The loss of buffering capacity is caused by improper water changes. Typically, this is caused by adding water to the aquarium to replace that lost by evaporation, but not actually removing old water from the tank during the water changing procedure. Removing old water from an aquarium is a very important part of the water change process. Failure to remove some of the old water at each water change allows organic acids to accumulate. These are produced by the fish and bacteria in the system and are normal. However, when the old water is not removed, the acids use up the buffering capacity of the water (measured by total alkalinity). When the total alkalinity falls to zero, the pH plummets, killing the biofilter and causing old tank syndrome.
Typically, a tank with old tank syndrome will have high ammonia levels (often higher than 10 milligrams per liter), little or no alkalinity, low pH (below 6.0), and high total hardness (several hundred milligrams per liter). In this situation fish should be moved to a separate holding tank, the aquarium should be totally broken down, everything should be thoroughly cleaned (including the biofilter), and then the entire system set up again as a new system. Because it is a new system without an established biofilter, you will need to monitor ammonia levels and do water changes as ammonia accumulates, until the new biofilter is established.

Anemia
Anemia is a condition in which the number of red cells in the blood is low. The most obvious sign that fish are anemic is very pale gills. Observant fish owners may notice this. Although not a common finding, many things may cause anemia. These include various infections and folic acid deficiency, which has been reported in channel catfish. Longterm exposure to nitrite in the water may also lead to anemia. You should consult your veterinarian or other fish health professional if you suspect anemia is a problem in your fish.
Leeches are parasitic bloodsuckers that may carry various blood parasites. Because they consume blood themselves, a heavy infestation with leeches can cause anemia. This does not necessarily mean that blood parasites are present. Aquariums and ponds usually become infested with leeches by introduction of an infested fish or plant. There are some approved treatments, but avoiding leeches is best, and depopulating infested aquarium or pond fish and restarting the system is very effective.

From: http://www.merckvetmanual.com/pethealth/exotic_pets/fish/disorders_and_diseases_of_fish.html