Why fish get bacterial infections We all know the water in our aquarium is full of bacteria. Most of it is good (like nitrifying bacteria), but some can be bad and harmful to marine animals. The bad kind 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, many times the immune system is still able to fight it off without the aid of antibiotic medication. 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 exit wounds left by feeding ich trophonts can get infected. Poor water quality can fuel bacteria levels. Also, anything that lowers the fish's overall immune system makes infection more likely. Stress (ex. fish fighting), malnourishment, or if the fish is battling an ongoing parasitic infestation (ex. ich) makes a "secondary" bacterial infection possible. For example, back when I practiced "ich management" it seemed periodically I would have a fish develop “popeye” 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 fish and more capable of overcoming potential infections. Gram positive vs. gram negative: The majority of bacterial infections seen in established fish are caused by gram positive bacteria. These can mostly be dealt with successfully by the fish’s natural immune system; in fact, in some cases symptoms are barely noticeable. On the other hand, gram negative bacterial infections are more often seen in newly acquired fish, and are more likely to require antibiotic treatment. 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. Unfortunately, there's no easy way of visibly differentiating gram positive from gram negative (without taking a sample and then using a staining process.) The belief that, "if it looks really bad or is widespread" denotes a gram negative infection is oftentimes true, but not 100% of the time. So, what does a bacterial infection look like? Sometimes it's self-describing: Popeye/cloudy eyes, fin & tail rot, dropsy (bloated fish), etc. But some other bacterial infections, such as furunculosis, hemorrhagic septicemia, etc., can have varying visible symptoms. Any "redness" or open sores/wounds on a fish should be viewed with suspicion. Also, a white film or "fungus" looking growth can denote a bacterial infection. However, these physical symptoms can also mean something entirely different. For example, redness around the gills is a symptom of ammonia burn, while a white "cauliflower-like" growth on the fins & spines is most likely just Lymphocystis, a harmless virus found in many fish. 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. How to treat a bacterial infection: 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 quarrelling fish before cuts/wounds get too serious. Utilizing proper nutrition (i.e. nori, foods high in protein), and soaking fish food with vitamin supplements (examples: Selcon, Zoecon, Vita-chem; or even Omega-3 fish oil - props to Paul B for the fish oil recommendation). These will help boost your fish’s natural immune system. Utilizing a fish QT - to prevent parasites and other nasties (including harmful gram negative bacteria) from being introduced into 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. Running a UV sterilizer 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. Below is a list of antibiotic medications you can use. It is not a comprehensive list by any means, but just some readily available options. Whatever medication you go with, be sure it contains broad spectrum or wide spectrum antibiotics capable of treating both gram-positive and gram-negative bacterial diseases. Kanamycin (ex. Seachem Kanaplex) Nitrofurazone (ex. Furan-2) Nitrofuracin Green Powder (Great for treating “red sores” commonly seen on butterflyfish and some angelfish. It is also useful for healing wounds and treating ammonia burn.) Triple Sulfa Powder (Good broad spectrum antibacterial medication; but IME can be harsh on certain fish.) Erythromycin & Minocycline combination (ex. Maracyn 1 & 2). If you are confident it is just a gram positive infection, then API E.M. Erythromycin is a good choice. For really bad infections, I advocate combining metronidazole (ex. Seachem MetroPlex), Furan-2 and Kanaplex to achieve a very broad spectrum of treatment. Props to “hedgedrew” for enlightening me of this. More info about specific antibiotics: http://www.americanaquariumproducts.com/AquariumMedication2.html 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, or any other medication for that matter. 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. 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. Some advocate just applying antibiotic ointment to the affected area(s) topically, but I have zero experience doing that with fish. 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.