Bacterial infections. All bacteria are opportunists; some will have invaded the tissues of a fish corpse within half an hour of death. But not all bacteria are pathogenic, that is, capable of causing disease. And not all pathogenic bacterial strains are virulent, that is, capable of killing a host. Two prevalent strains are Æromonas and its kin, the bacteria called Æromonads, and Mycobacteria. Strains of Æromonas hydrophila are particularly prevalent in warm-water fishes and amphibians and can cause gastroenteritis in humans.
But bacteria are everywhere; they cannot be kept out of the aquarium. Here's an excellent post from Mike Duskis (aka MADman) at Aquaria Central:
"The common pathogens in the environment are not sufficient in themselves to cause disease. If they were, every animal would be ill all the time. An animal becomes ill when something changes in its body or in its environment that gives the pathogens an opportunity. Too much of our medicine focuses on eliminating the pathogens while ignoring the environmental factors ...factors such as water and air quality, plumbing, and the health of our neighbors have a great deal of effect on our health. Pathogens will always be with us, but if our environment is healthy, we are much less likely to become ill. "The same goes for aquarium fish, but perhaps to an even greater degree, because we have so much control over their environment. In the last four years or so, the only disease I've seen in my tanks has been fungal fin rot — and that has always been brought on by cichlid aggression, clearly an environmental factor. Aggression aside, my fish do not get sick. They do not get sick because their environment is balanced and stable.
"I suppose you could say that I'm on a crusade for thinking about fish wellness rather than fish disease. Yes, we need to treat infections when they arise, but we should also think about the environmental causes of those infections. If we do not, we will be forever dependent on medications which, in my experience, can do great harm to the aquarium ecosystem and result in more disease."
You can be sure that many bacteria that could have a pathogenic effect on a fish with a compromised immune system are already harmlessly present in the aquarium. The main reservoir of latent bacterial infection is not the gravel that you might be vacuuming so diligently, however, but the other fishes. The bacteria involved are ubiquitous, but they aren't ordinarily virulent, that is, not until environmental stresses depress a fishes' immune system. By the time bacterial populations have overwhelmed a fishes' immune response to the extent that it is showing acute symptoms, such as bloating or impaired balance, the sick fish has become a major source of infection for its tankmates.
The commonest vectors for infection are all from fish to fish. The most common source of infection is from eating a recently dead fish; always try to avoid this source by promptly retrieving any losses. It's good practice, too, to remove any fish that is showing terminal symptoms before it actually dies. When advanced mycobacteriosis and other bacterial infections have caused skin ulcerations and lesions, these are another source of infection for tankmates.
Bacterial infections aren't "curable." The fishes' own immune system may kick in and swamp the infection by forming lymphocytes, very much as terrestrial vertebrates like us also do. In the meantime, the best any anti-bacterial medication can do is slow down the replication of the invaders, which gives the immune system time to get activated.
Products marketed for the aquarium hobby may be called "bacteriostatic" or "bactericidal." A bacteriostatic ingredient stops bacteria from dividing and multiplying. A bactericidal ingredient kills them. For an ingredient to be labelled "anti-bacterial", it need only slow the division of bacteria, no more. Microbiologist Dr. Beverly A. Dixon (California State University) gave an introductory survey of these functions of antibiotics in an article "Antibiotics: how do they work?" that was origionally published in Aquarium Fish, Feb 1992.
An examination of aquarium fishes and their carrying water in 2009 showed that the routine use of antimicrobials in commercial fish culture and in their carrying water, to suppress developing bacteria, form sustained selection pressures that resulted in multiply-resistant strains; just over half the Aeromonas isolated were tolerant to about fifteen antibiotics (see this abstract of the article by David W. Verner-Jeffreys and team). Questionable effectiveness doesn't keep various preparations that contain "anti-bacterial" ingredients off the shelves at your LFS.
Tetracycline is the classic example. In the US it is routinely added to animal feed as a growth enhancer, over the ineffectual objections of the F.D.A. (It seems to depress certain gut bacteria that destroy the vitamin B12 created by other bacteria.) Does routine use of antibacterial drugs promote growth in fishes, too? I don't know the answer to this, but the annual use of antibiotics in American agribusiness represents some eight times the amount used annually to fight infection in humans. The exact figures are obscured because the chicken/pork/beef industries join ranks with veterinary drug companies to form the Animal Health Institute, to protect as "business secrets" actual figures of drug production and use.
Tetracycline is still being sold to you as a medication for tropical fish. But most any bacteria you'll meet with have evolved resistance to it.
It's not easy to establish, even in a double-blind clinical trial, whether a medication is genuinely effective. Even more difficult to prove that it is not effective. Unproven and even ineffective "anti-bacterial" remedies will continue to find a ready market. I'm skeptical, as ever. You'll have to decide for yourself whether you can have any effect in helping a fish recover from a bacterial invasion. Do remember that the purpose of antibiotics is to stall the reproduction of the bacteria while the host's immune system assembles the appropriate ammunition. It's the immune system that actually cures the fish, not the medication.
"Pre-emptive" use of antibiotics is always an error. They have recently crept into hand lotion and kitchen towels; casual misapplication of antibacterial drugs has spurred the evolution of resistant bacterial strains.
The aquarium hobby isn't alone to blame here, but when I first wrote this, in 2001, resistant Pseudomonad and Aeromonad bacterial infections were reported to be commonplace in fishes raised in Hong Kong and Taiwan. Singapore and Bangkok, on the other hand, have begun to address the problems raised by hatcheries' routine use of antibiotics as a prophylactic measure.
Dilution. Apart from the issues of resistant strains, drugs that are effective when injected may be less than effective when diluted in a bath. My own disheartening conclusion is that the bacterial diseases of fishes, like their viral infections, are largely untreatable in the home aquarium.
Nevertheless, it's worth looking into the kinds of bacteria most usually held responsible for problems.
Untreatable bacterial infections? This "defeatist attitude" just isn't what anyone wants to hear, and what's worse, it sells no products! Though most of us have firmly-rooted illusions about the effects of our pet "anti-bacterial" patent medications, you might want to keep these words in mind, as you cruise the shelves at the LFS to see just what "cures" for bacterial diseases corporations are offering you.
"Columnaris Disease." Flavobacterium (Flexibacter) columnaris causes headaches for fish farmers raising Channel Catfish under crowded and stressful conditions and prolonged summer heat; 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; when Neon Tetras imported into the French aquarium tradefrom the Far East were experiencing mass mortalities in 2000, the muscle necrosis and other symptoms presented looked like infection of microsporideans, but an Asian strain of Flavobacteria columnare turned out to be responsible. An ordinary aquarist is without resources.
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. Encourage bacterial diversity in the aquarium. This may be unfamiliar advice. I encourage bacterial diversity in several ways. I add a pint of water drawn from a fish-free plant nursery to each aquarium, from time to time.
I also vary water temperatures, keeping within comfortable tolerance ranges of the fish, of course; after a couple of months at 77°F, for instance, I might re-set the heater to 74°F for six weeks or so. Why? Well, our constant thermostatically-controlled temperatures favor the success of whatever strains of bacteria are the most ideally-suited to that particular temperature. Mycobacteria marinum reproduces best in the lab at a steamy 33°C. Varying the temperature may avoid inadvertent culturing of dominant strains of bacteria. Seasonal variation of temperature — within the limits tolerated by the fish — seems to have a protective effect: Dr. H. Reichenbach-Klinke was suggesting in the 1960s (Diseases of Fishes, TFH, 1965, p. 23), "Obviously the adaptability of the fish has to be considered, but attempts to protect them by varying their temperature seem to be well worth while." I think we've ignored this sound old-fashioned advice, which I first read in William T. Innes' Exotic Aquarium Fishes. As far as tropical fishes are concerned, the difference between surface waters in a slow-moving stream at dawn and at mid-afternoon may be as great as any seasonal differences. "Night is the winter of tropical waters," I recently read.
I also encourage a healthy biofilm, to let "good" bacteria crowd out "bad." I encourage healthy intestinal fauna with a varied diet that includes plenty of roughage from cellulose and chitin. Meanwhile, apart from aquarium situations, the Center for Disease Control, Atlanta GA, recommends a 10% solution of household bleach to sterilize equipment contaminated with mycobacteria.