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Bacterial Diseases in Marine Aquarium Fish

Identification, Causes & Treatments


The world is full of bacteria, in fact, our world would not exist as we know it without them. In 1884 a Danish physician, Christian Gram, discovered that bacteria could be separated into two distinctive groups, gram-positive and gram-negative. Using a particular staining process the bacteria could be determined either gram-positive or negative, depending on whether they retained (positive) or lost (negative) a violet color during this process. Most bacteria that cause disease in marine fish are gram-negative. Ones most commonly associated with these infections are of the genus Pseudomonas and Vibrio, as well as Myxobacteria.

Diagnosing bacterial diseases and why they occur is not always a simple, straight forward task. Bacterial infections can stem from many causes, even combinations of contributing causes, and can be topical (external - i.e. fin & tail rot and ulcer diseases), systemic (internal), or both. Healthy fish have a strong, natural resistance to bacteria and can usually fight it off on their own, but weak, sick and/or stressed fish can be very susceptible to this illness. The tiny open pores of the lateral line or open wounds are most often the point of entry for the onset of internal infections, but the soft tissues of the gills are also a prime entry point. Contributing factors that can open the door to this disease are:

  • Poor environmental conditions. If conditions are poor enough, the bacteria can bloom and overrun even the healthiest of fish. White cloudy water with the presence of sores on the fish can be signs of this problem.
  • A secondary infection resulting from having another disease. An excellent example of this is the open sore left by a parasitic infection, such as Cryptocaryon irritans. In many cases the fish has been cured of the parasite, but the resultant infection from bacteria entering the fish's body via the break in the skin has caused death.
  • Poor nutrition.
  • Injury, such as open wounds, cuts or scrapes.
  • Harassment.
  • Stress.
  • Weakness.
  • Old age.
  • Introduced into the aquarium by means of a contaminated water source. Don't laugh, this happened to us once when we used tap water. The local water supply had a bacteria outbreak after allowing the chlorine levels to drop too low.
  • Ingestion. Example - Fish eating the flesh of another infected fish.
The common signs can be one or a combination of the following:
  • Reddened and frayed fins, or red streaks through the fins. Red fins are often a sign of "ammonia burn", which can be caused by poor packing procedures during shipping.
  • Disintegration of the fins, i.e fin and tail rot.
  • Redness around the area of the lateral line, often seen as streaks or blotches.
  • Open sores on the sides of the body and near the fins.
  • Bloody scales at the base of the fins.
  • Rapid breathing.
  • A gray film over bulging eyes (Pop Eye).
  • Listlessness or lethargy.
  • Loss of appetite.
  • Abdominal swelling or bloating. This is usually a sign of a bladder infection, often a common problem with Wrasses that burrow in the substrate.
Bacterial diseases usually do not kill fish in a couple of days, especially larger ones, but more over a one to two week period. However, there are some viral strains that may do so. In any case, the prompt isolation of infected fish and treatment with an antibiotic in a QT (quarantine tank) is important. Even minor topical infections can progress quickly, and once the disease is in the systemic stage it affects the internal organs. The fish stops eating, the respiration rate increases and the fish eventually lays on the bottom and dies.

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