I'm not the writer of this article.
Columnaris Disease." Flavobacterium (Flexibacter) columnaris causes headaches for fish farmers raising Channel Catfish under crowded and stressful conditions and prolonged summer heat but doesn't so often affect aquarium fish. Symptoms include "gill rot" and so-called "saddleback" lesions, pale areas across the nape. "Columnaris Disease" can also have insidious internal effects that are hard to cure. There's a good downloadable article on "Columnaris Disease" at the Southern Regional Aquaculture site, and Discus Page Holland also has a good article on "Columnaris Disease" which includes treatments with saltbath dips or potassium permanganate.
These bacteria are hard to isolate in the lab, technicians say, and probably don't ever naturally occur isolated from other bacteria and fungi. The same strains of bacteria and fungi that can be isolated in smears from affected fish can also be found in cultures taken from neutral areas of the aquarium, for example from decaying flake feed. They are often found perfectly harmlessly growing on healthy fish. So whether they are responsible for "mouth fungus" or not is arguable. No species of fish is especially resistant. You've probably noticed that fish are rarely troubled with fin rot or mouth fungus in your balanced aquaria--— it's much more likely to be found among new fishes in Quarantine. Fish with bacterial/fungal conditions have to stay in Quarantine to recover, even if it takes longer than a month.
How does bacterial resistance come about? When you use antibacterial products you don't kill all bacteria, just the susceptible ones. The balanced community of bacterial strains is disrupted at the first use of the "antibacterial" ingredient, but the surviving bacteria swiftly take up the newly-available space and nutrients. The next antibacterial round will be less effective, because you are conducting what amounts to a long-term experiment to select for resistance to the particular anti-bacterial agent you are using. The resistant bacteria were probably already part of the richly-mixed bacterial community as a small potential "founder population," but they were unable to establish themselves until competing microbes were destroyed. Now their neighboring bacteria may even borrow nucleic acids that code for resistance.
Taking the broadest long-term view, it looks as if scattershot antibacterial medications do more harm than good. Now consider an effective antibacterial agent, but which has been diluted in the aquarium water. At this low concentration, an agent is unlikely to have the kind of effect it might have had in a Petri dish in the laboratory. Most successful antibiotic treatments of fish require intramuscular injections: a treatment we're mostly just not prepared to administer, and not practical with small aquarium fish anyway.
Some precautions. If cure is largely impractical, then controlling pathogenic bacteria is all the more important. Prevention of bacterial disease is the strategy behind some key tactics:
You can take some precautions to minimize the transfer of pathogenic bacteria. Avoid unnecessary transfers of fish from tank to tank. Isolate fish that are showing early symptoms of bacterial infection. Don't wait for a fish to develop terminal swim bladder disfunction, edema so severe that the scales stand up, popeyes or ulcerating lesions that trail mucusy slime into the water. Remove a fish with symptoms of bacterial disease at an early stage. Even if you can't bring yourself to euthanize it, at least isolate it. And don't wait for a dying fish to expire in the aquarium. A dying fish is increasingly a reservoir of bacteria and weak parasites, some of which will be released into the water when it dies. The viscera and carcases of dead fish are smorgasbord for their tankmates, creating a most effective vector for disease transmission. In a related note, at salmon hatcheries, a substantial reduction of "tb" infection was achieved simply by eliminating uncooked salmon viscera from the diet of fry. So never allow a dead fish to be picked over by other fishes. I'm unsettled to see nibbled corpses in the tanks at the LFS for just this reason.
Isolate asymptomatic carriers. You may medicate a beloved sick fish if you wish, but don't return to a community aquarium a fish that has "recovered" from symptoms of bacterial disease once its outward symptoms have been alleviated. "Dropsy" is a case in point. Sometimes a fish recovers enough from a bout of severe ascites to lead an outwardly-normal life. Then the "cured" fish is returned to the community aquarium, where it may become a sub-clinical carrier of bacteria, free of visible symptoms. A sub-clinical carrier remains a source of infection for all your other fish. When it dies quietly among the plants, a couple of months after the episode of "dropsy," the two events may not seem connected.
Initial quarantine, even a full four weeks' time, may not be long enough to identify weakened fish that are bacterial carriers before they enter your system. It's quite probable that all your fish have already been exposed to a variety of bacteria that could be pathogenic, given the right circumstances.
Encourage a low-stress environment, to keep immune systems responsive. Keep levels of dissolved organics and metabolic end-products low.
I am now treating the tank with Melafix as well.
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