Bacterial Diseases in Marine Aquarium Fish

Warty Frogfish

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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, e.g., fin and 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 through a contaminated water source. 
  • Ingestion. E.g., 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.
  • The disintegration of the fins, e.g., 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 moreover a one to two week period. However, some viral strains 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.

Most of the time only one or two fish in an aquarium at any given time show signs of an advanced case of bacterial infection. However, if the environmental conditions in the aquarium are poor enough, it can severely affect all of the fish. If this is the case it is best to perform a major water change, treat all of the fish in a QT and give the aquarium a major cleaning overhaul. This, as well as adding a UV sterilizer can sometimes help to prevent the perpetuation or re-occurrence of the same problem in the future.

Using the Right Antibiotic

Many bacteria are naturally resistant to specific drugs, so using the proper type of antibiotic is of the utmost importance. For example, gram-positive bacteria are often unaffected by tetracycline and streptomycin, while gram-negative bacteria may be unaffected by ampicillin, penicillin, erythromycin, and sulfa drugs. Some bacteria may also be unaffected by the usual dose but are susceptible to a higher dose.

Here is another good example. Mardel Laboratories makes Maracyn and Maracyn 2. Maracyn is erythromycin based and is a gram-positive bacteria treatment. Maracyn 2 is mincycline hydrochlor based and is a gram-negative bacteria treatment, as well as sufficiently absorbed through the skin to treat internal infections. Since most bacteria that cause disease in marine fish are gram-negative, and can quickly become systemic, Maracyn 2 would be the most effective choice. Mardel also makes and distributes Maracyn Plus, which is a broad-spectrum antibiotic for controlling the bacteria that cause mouth fungus, fin and tail rot, Popeye, dropsy, and ulcers.

You have to take into consideration that you will not be able to determine the exact type of bacteria you are dealing with, mutated strains can be produced, and both bacteria might be present. Broad-spectrum antibiotics, ones that kill both gram-negative and positive bacteria, such as neomycin, chloramphenicol, nitrofurazone (Furan-2) based products, skin absorbed kanamycin sulfate-based antibiotics such as Kanacyn/K-Mycin, and secondly tetracycline, are usually effective treatments. Aquatronics makes an ultra-wide spectrum antibiotic blend of nitrofurazone and kanamycin called Spectrogram that may suit your needs. If after a few days of using a particular antibiotic there appears to be no or little improvement in the health of the fish, try another one. These are just a few of the many available products on the market, so do your research to ensure that the medications you choose are safe to use in conjunction with each other if you decide to mix them.

The overuse of antibiotics for treating fish is the same as with people. The repeated or continued use of antibiotics creates selection pressure favoring the growth of antibiotic-resistant mutants. Our advice is not to use antibiotics as a cure-all treatment whenever you feel the fish "might" need it, but only when it is necessary.

Overall, the best method for treating bacterial infections is orally rather than topically (treating the water), or using a combination of both. Oral feeding can be accomplished by mixing an orally expect-able antibiotic into some highly palatable blended frozen food. Some antibiotics have instructions for oral applications, just read the labels for information. Keep in mind that if a fish has stopped eating altogether, oral feeding is not possible and chances are its days are numbered. If you do not want to hassle with mixing your own medicated foods there are various types available that you can purchase.

Since many antibiotics kill gram-negative bacteria and the biological nitrifying bacteria of an aquarium are also gram-negative, we recommend that you do not treat the main aquarium. Whether it is a fish-only or a reef tank, a mild dose of antibiotics can greatly weaken the biological growth, a strong dose may kill it altogether, while invertebrates and marine algae may be killed off or affected as well. If you are compelled to use these or any other medications to treat the main aquarium, beware!

If you do use an antibiotic on your main tank, you might consider dosing the tank with a good Nitrifying Bacteria Tank Starter after the antibiotic treatment is complete.

The bottom line is whatever type of medication you use, make sure you get one that will treat the ailment you are dealing with. Follow the recommended dosages and directions for use on the product labels, and when in doubt, consult with your local fish store professional for assistance.

Article Sources
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  1. Bacterial Diseases of Fish. Merck Veterinary Manual

  2. Bacterial Diseases in Aquaculture. Merck Veterinary Manual