Bacterial infections affecting aquarium fish are relatively common. Aeromonas and chondroccus infections are particularly common. Poor tank and water sanitation is usually to blame, though introduction of contaminated water and drastic changes in water temperature can also contribute. In addition, malnutrition, stress or harassment, old age, and injuries can leave fish more susceptible to bacterial infections.
A definitive diagnosis of a bacterial infection requires a sampling of bacterial culture from an affected site, which is of course not generally practical. You can rely on visible symptoms to make a determination, and then check with your veterinarian about whether you should proceed with an antibiotic treatment.
Symptoms of Bacterial Infections in Fish
If your fish have a bacterial infection, you can expect to see one or a few common indications. These include ragged or reddened fins, fin or tail rot (meaning they show signs of deterioration), red streaks or blotches around the lateral line, ulcerations, bleeding scales, enlarged eyes, a grayish film over the eyes, accelerated breathing, inactivity, diminished appetite, and abdominal swelling.
Treating Bacterial Infections in Fish
Left untreated, bacterial infections cause renal or other damage and they are typically fatal within a few weeks. The smaller the fish, the more quickly they tend to die from an untreated infection. All bacterial infections--even seemingly minor, superficial ones--quickly become a serious concern in fish.
The treatment for bacterial infections in fish is an antibiotic. Using an appropriate product that is effective against the particular strain is important; otherwise, the infection will continue to spread, perhaps even at an increased rate. This is why you should consult your veterinarian before deciding on an antibiotic for fish. Sometimes a broad-spectrum antibiotic that kills many gram-positive and gram-negative bacteria, such as Fish Mox, is called for. If your fish are affected by a gram-positive bacteria, it is unlikely that a tetracycline or streptomycin antibiotic will work. Penicillin antibiotics and sulfa drugs are less likely to work against gram-negative bacteria.
Most fish antibiotics conveniently allow you to treat the entire tank at one time. When using an antibiotic, it is important that you complete the prescribed regimen. In many instances, visual signs of the infection will be cleared up before the bacteria are completely eliminated.
References
PetMD
http://www.petmd.com/fish/conditions/skin/c_fi_aeromonas
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