Are you certain that you have a bacterial infection? Without a sample from a fish you can't be certain that it's not a "bug" like velvet or brooklynella but usually bacteria accompany these infections. The treatment of parasites vs. bacteria is very different...why waste time and money and watch fish die because you are treating the wrong thing? If you have a microscope and a bit of training on fish sampling, you can try to do diagnostics yourself. If you want to try to throw antibiotics into a tank or try some medicated food, it might work, or it might now. It might cause delays and more problems. Ideally, when treating ornamental fish we recommend referring to a licensed aquatics veterinarian for antibiotic use, especially for situations where a large number of animals are exposed. While folks reading this article probably aren't fish farmers, there may be groups of susceptible animals in your display tank. With a group of animals, a veterinarian can usually identify if the cause of disease is bacterial and can take a sample to grow in order to identify the antibiotics which will work specifically on that infection (it’s called doing a culture and sensitivity for a bacterial infection). If a sample is not possible, an educated guess and appropriate drug can be provided. This reduces that amount of cost; less medications wasted and less fish loss with fish returning to healthy faster. It also reduces the instance of resistance selection so that you don’t make super bacteria, which also cost a lot to fix later. Overall, treating with veterinary guidance usually saves everyone time and money in the long run. Individual treatment of your ornamental fish, we like to use the 1.5 inch flame angelfish example, might intuitively not sound like a good enough reason to call the vet, but if this sick fish has the potential to get your 11 other fish sick as well, there is the potential for major loss both emotionally and economically. Here is a quick guide to help you understand bacteria in aquariums and how to potentially treat.
Antibiotics are gram positive or gram negative and bacteriostatic or bactericidal
Most bacterial diseases of fish (both salt and fresh) are generally caused by gram negative species of bacteria (Aeromonas sp.most commonly in fresh, Vibrio sp. in salt)
Bacteriostatic drugs prevent bacteria from replicating and let the fishes immune system fight the current infection. These drugs need to remain at a certain concentration in exposure to the bacteria, in the tank environment itself to deal with external infections or in the blood or tissues of the fish. In the case of in the tissues of the fish, the drugs are ideally maintained by injections by some drugs can achieve a therapeutic level through emerging the fish in water of a certain concentration of that antibiotic( what these values are is still unexplored for most fish/drugs). A few drugs which are more readily absorbed are nitrofurazone, erythromycin, and TMP-sulfa.
Bactericidal drugs kill the bacteria at a certain concentration. These drugs are often given for short durations at intervals (again not very well researched). All of these drugs have the potential to disrupt a developed biological filter and therefore the balanced ecosystem we try to maintain. There are many organisms contributing to your aquariums balance which may be inadvertently killed when using these medications. Nitrifying bacteria are gram negative and disrupted by drugs like erythromycin causing most tanks to undergo an ammonia spike because it’s not being converted to nitrite by the bacteria anymore. Use a quarantine tank if possible.
While it's not always possible to catch the fish easily, think things through and don't do anything rash and put other fish in danger ( for example attempting to catch a fish and disrupting the rockwork stressing all of the fish and causing scrapings on other fish which may get infections)
Again, ideally you would have a bacterial culture and antibiotic sensitivity done by a veterinarian or a lab to determine at least the presence of bacteria and how to kill it. This is how veterinarians want to approach cases of potential bacterial disease and it makes sense for large aquarium collections and aquacultured fish but also for private collections which have the potential to lose everything. This, we realize, is often unrealistic for individual ornamental fish in a private aquarium and the process of handling the fish to collect the sample may be more detrimental than beneficial. Collecting an accurate sample of the disease causing bacteria from a live fish is also difficult as the marine environment is full of ambient and non pathogenic bacteria but guidance from a veterinarian trained in aquarium science and understanding of the nature of bacterial disease in aquariums will allow for the right antibiotics to be selected and the right treatments (through feed, dips, injections, quarantine, etc.) to be performed. This makes sense in most cases for reducing overall stress (both for fish and people) and costs.