Bacteria are present everywhere on both land and in the water and are normally classified as either 'good' bacteria or 'bad' bacteria. The good bacteria in the aquarium consist of the ones that make up the biological filter. These bacteria break down ammonia and nitrate so that it can be removed from the water. Without good bacteria most life would not be able to exist.
The bad bacteria are often bacteria that are normally present but don't cause problems until the fish is injured, stressed or suffering from another disease. These bad bacteria take advantage of the compromised animal's weakened immune system and reproduce extremely quickly, creating the resultant sicknesses and problems.
The key to dealing with any bacterial infection is early recognition and treatment. Of course, prevention through careful introduction of new fish, plants and water, as well as maintaining a healthy, stress-free environment for your fish, is still the best course of action.
This two-part article will assist you in the identification and treatment of some of the more common bacterial infections in fish.

Fin rot

Signs: Fin rot often begins with red streaks in the fins that are soon followed by abnormal lightening of the edge of the fin and then a fraying or rotting of the fin membrane and edges. This infection can lead to a complete rotting of the fin and can spread to the body and lead to death.


Transmission: The bacteria can be transmitted through the water from open ulcers, feces of infected fish, or through the consumption of infected, dead or dying fish in the tank.

Predisposing factors: This disease is not highly contagious, however it is often associated with poorly kept or dirty tanks with poor water quality. Any dead fish should quickly be removed and disposed of. Diseased live fish should be removed to a treatment tank.

Treatment: The antibiotics kanamycin or erythromycin are sometimes successful. All infected fish should be handled with care to prevent transmission to humans through open wounds or cuts

Skin ulcers
Signs: Ulcers usually show up as raw opened areas on the skin. They often have reddened edges and may be associated with other symptoms of systemic infection or disease.

Cause: Aeromonas, Pseudomonas, Mycobacterium and Vibrio
Transmission: The bacteria are commonly found in the water and will invade a wound or skin injury on an otherwise healthy fish.

Predisposing factors: Previously damaged skin, poor water conditions, and stress can all lead to an increase in ulcers.

Treatment: Antibiotic baths coupled with anti-fungal baths containing phenoxyethanol are the most common treatments. Improving water quality, reducing stress, and decreasing the risk of injury are all very important in reducing the incidence of ulcers.

Vibriosis

Vibriosis is most often found in marine or brackish water fish but can occasionally be found in tropical species as well.

Signs: There are two forms of vibriosis.
Acute Form: Death may occur suddenly, before any signs are noticed. Symptoms may include increased respiration, loss of appetite, lethargy, skin hemorrhages, and death. Post mortem exams may reveal enlarged internal organs, but without a diagnostic test, this disease is difficult to distinguish from other bacterial infections.

Chronic Form: Exophthalmos, ulcers, and intestinal inflammation in fish that have died.

Transmission: Fish contract the bacteria Vibrio anguillarum through open sores or feeding on dead fish that died from the disease.
Treatment: The best treatment includes the oral antibiotics chloramphenicol or furazolidone.

Summary

These are just a few of the most common bacterial infections that can infect fish. After reviewing the list of infections, it is very clear that most bacterial infections are caused by a few similar situations, and that following a few basic precautions can prevent most of these infections. The precautions include maintaining excellent water quality at all times, quickly removing any dead or diseased fish from the tank, treating all diseased fish, never introducing diseased or sick fish into your community tank, and preventing injuries from fighting or unsuitable habitat. If these basic guidelines are followed, bacterial infections will be a very rare occurrence in your tank.
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Fish Tuberculosis

My goldfish tank got tuberculosis from a contaminated holiday food block. One of the fish was blind and not getting enough to eat so I'd been putting blocks in to feed him up a bit. Anyway, I was away for two days and when I came back ALL my goldfish had tb. I treated them with a 50% water change, aquarium salt, and melafix. Two died, two survived. I'm not sure if this was the best way to treat tb, but that's what I did...

No, no. I meant which of the diseases you needed a source for. But, then again, it would be nice to have sources for all of them.

Whitespot (ich) lives in the water so if your fish get it, you need to treat the whole tank and not just the affected fish.

When my fish have had ich I have done a fairly big water change, say 50% and then dosed them with King British White Spot Control. There are many ich meds out there but this is the one I used. Also raise the temperature a little if you can because this will speed up the ich lifecycle.

The important thing about treating whitespot is that you need to have patience. There is no quick cure. Once your fish have the spots there is nothing that can be done about them, what you're trying to do is kill off whats in the water.

Treatment should continue for at least 4/5 days after the spots have cleared to make sure that the ich in the water is once again dormant.