Bacterial Infections

Why fish get bacterial infections

What You Need To Know:

* Infections can be caused by either Gram‐positive or Gram-negative bacteria. Gram-negative infections are typically more virulent, and more common in marine fish.
Bacterial infections are often “secondary” to a preexisting parasitic or worm infestation.
Poor water quality, open wounds and nutritional deficiency (which lowers the natural immune system) are all possible contributing factors of infection.
Best treatment for a bacterial infection is a broad-spectrum antibiotic: Antibiotics

Additional Information

We all know the water in our aquarium is full of bacteria. Most of it is good (like nitrifying bacteria), but certain strains can be harmful to aquatic animals. This “bad” bacteria is usually kept at bay by a healthy fish’s natural immune system. Or if the fish becomes sick and displays symptoms of a bacterial infection, sometimes the immune system is still able to fight it off without the aid of antibiotics. So if you stop and think about it, these bacterial infections in fish are akin to our own never-ending battle with germs, viruses, and of course, infections. ;)

Many factors make a fish more susceptible to infection. First, a cut or open wound is usually required for infection to set in. Even tiny “bite marks” left by feeding parasite trophonts can get infected. Poor water quality is also usually a key ingredient which increases the odds of a bacterial infection. Also, anything that lowers the fish’s overall immune system makes infection more likely. Stress (e.g. fish fighting), malnourishment, or if the fish is battling an ongoing parasitic or worm infestation makes a “secondary” bacterial infection more possible. For example, back when I practiced “Ich management” it seemed periodically I would have a fish develop a cloudy eye or some suspicious red mark. Now that I quarantine (QT) and prophylactically treat all my fish, I almost never see anything like that once the fish enters my display tank (DT). Disease-free fish are healthier and more capable of overcoming potential infections.

Gram positive vs. gram negative: Unfortunately, the majority of bacterial infections in marine fish are caused by gram-negative bacteria. These are typically more virulent than infections caused by gram-positive bacteria. When a fish does have a gram-positive infection symptoms may be so mild that they are unnoticeable. The only way to differentiate between the two is to take a skin scrape of the affected area and gram stain it (more info). Gram-positive bacteria stain blue, and gram-negative bacteria stain pink.

Gram-negative infections are nothing to play around with. In fact, some fish can die within 24 – 48 hours of showing symptoms due to the aggressive nature of some gram-negative bacteria. These strains can easily overwhelm the fish’s natural immune system. So, what does a bacterial infection look like? Symptoms may include:

  • Any “redness” or open sores/wounds on a fish should be viewed with suspicion.
  • A white film or “fungus” looking growth can denote a bacterial infection.
  • Frayed fins / fin & tail rot.
  • Cloudy eyes
  • Bloating can denote an internal bacterial infection.

However, it is important to note that the symptoms listed above can also mean something entirely different. For example, a white “cauliflower-like” growth on the fins & spines is most likely just Lymphocystis, a harmless virus found in many fish. Redness around the gills is a symptom of ammonia burn, while open red sores on the fish could be Uronema. So, doing proper research and not just lumping everything into one category is vital. Whether or not a bacterial infection is contagious is highly dependent upon the strain you are dealing with and the conditioning of your other fish. So, there is no easy answer.

Treatment Options – First off, there are many things you can do to prevent a bacterial infection from happening in the first place. Some of these include:

  • Maintaining a proper environment (i.e. clean water) for your fish to live in.
  • Separating two quarreling fish before cuts/wounds get too serious.
  • Utilizing proper nutrition (e.g. live foods, nori, frozen foods high in protein), and soaking fish food with vitamin supplements (e.g. SelconZoeconVita-chem). These will help boost your fish’s natural immune system.
  • Utilizing a fish Quarantine Tank – to prevent parasites and other nasties (including harmful gram-negative bacteria) from ever entering your DT. This will alleviate the possibility of a “secondary” bacterial infection popping up while the fish’s immune system is already compromised from battling parasites/worms.
  • Running a UV sterilizer or Ozone may help in certain situations, as that will lower the overall number of harmful bacteria found in the water column.

When to medicate: Sometimes all the vitamins, proper nutrition and clean water are just not enough. Sometimes a fish’s natural immune system needs a helping hand (like our own). When to QT and pull the trigger on using antibiotics is not an easy decision; it’s a judgment call.

As a general rule, I only pull & treat if: (a) The fish looks really bad or (b) It is a newly acquired fish showing signs of infection. The latter is an easy call for me as I QT all new fish anyway. Here is a list of antibiotic medications you can use:

More info about specific  antibiotics:

I also recommend giving the fish a 90 minute bath using Ruby Reef Rally en route to quarantine. Although it is not optimal to do so, you can combine antibiotics with copper treatment or Chloroquine phosphate. Since I do not use hyposalinity to treat Ich, I have no experience using antibiotics in hypo conditions. I do not recommend mixing Prazipro with antibiotics. It is important to remember that every medication you use depletes the water of oxygen. Combining meds just exacerbates this. Therefore, it is absolutely imperative to provide additional gas exchange when treating; by pointing a powerhead towards the surface of the water or running an air stone on high.

DO NOT overdose antibiotics; if in doubt, always underdose. Antibiotics can be harsh on and even kill certain fish; although appetite suppression is much more common. Antibiotics will kill some of the nitrifying bacteria in your bio-filter, but rarely wipes them all out to the point where you see an ammonia spike. However, for this reason and the negative impact antibiotics can have on corals/inverts, I strongly discourage their use in a DT.

More information below:

Use of Antibiotics in Ornamental Fish Aquaculture

Bacterial Diseases of Fish