Fungal infections

Fungal infections are much less prevalent in fish than bacterial infections. The likelihood is that any microscopic investigation of fungused areas would most typically show that the Saprolegnia "fungi" were confined to necrotic tissue, tissue that was already essentially dead or dying. Combinations of various bacteria and fungi are responsible for "fin rot" and "tail rot," for gill necrosis and body lesions, as well as the cottony hyphae of mouth "fungus."
 
Such infections often attack a single fish in an aquarium. They are not particularly communicable from fish to fish.
 
Treatments for all these conditions are similar. The essential is to catch the condition early, at the very first appearance of a white area at a fin edge, and move fast to correct conditions without delay. The pros always emphasize that only sick or immunologically compromised fish are susceptible to fungal infections. Stress from crowding, deteriorated water conditions, unnecessary handling or sudden drops in temperature can help lower a fishes' resistance. Normally, not every abrasion from being netted, nor every normal scrape or bruise leads to fungal infection.
 
Saprolegnia. The most visible culprits are members of a group of water molds collectively called Saprolegniaceae. Normally the Saprolegnia group are saprophytes, which means that they make a living breaking down organic plant and animal tissues and even decaying wood: in Latin "saprolegnia" signifies "the wood rotter." In this common mode of life Saprolegnia and its cousins among the water molds are a normal part of the biofilm and help recycle nutrients in the aquarium. The spores of Saprolegnia are always everywhere. But Saprolegnia are opportunists that can invade necrotic tissue, which is deprived of the normal supplies of blood and lymph that routinely carry away invading organisms and destroy them.
 
Though you don't see the fungal hyphae of Saprolegnia until they appear as a cottony growth, fungal symptoms such as "cotton-mouth" are actually secondary infections. They follow some primary trauma that isn't healing fast enough to keep ahead of germinating fungal spores. Healthy fish are able to resist Saprolegnia by the flow of mucus, which sweeps away spores, and by actively inhibiting factors found in normal mucus, which protect the fish until damaged tissue has been healed.
 
Mouth "fungus" used to be often attributed also to "columnaris," a name that alludes to one species (Flavobacterium [Flexibacter] columnaris) among a group of myxobacteria that are ever-present, both in aquaria and in natural waters. You'll find more about "columnaris disease" among the bacterial infections.
 
Finrot (or "tailrot") is a common disease, where the fin tissue erodes away from the edges and the fin gets ragged or splits. With spiny-finned fishes such as gouramis and cichlids, the fin may recede between the more resistant fin spines, leaving them to protrude like the spokes of a torn umbrella. In fin rot the eroding edge may have a whitish cast, where bacteria are joined by oocyte "fungi" like Saprolegnia. If the condition is ignored, the entire fin may rot away, and reddened areas of bacterial inflammation and sores can even appear on the fishes' body near the base of the fins.
 
Things should never get this far in your aquarium. You need to take action at the very first sight of finrot, at the thinnest white edge to a fin that doesn't ordinarily have one. If you catch finrot early enough, the thin membrane of fin tissue regenerates within weeks to months. Fin spines may not regenerate as well. Fins that have eroded to the base don't usually regenerate.
 
Multiple causes. I don't think it's useful to try to isolate "fungal" finrot from "bacterial" finrot. "Fin rot is a bacterial disease involving opportunistic bacteria such as Aeromonas, Pseudomonas or Flexibacter that abound in all aquatic environments. Secondary fungal infections are not uncommon," Frank Prince-Iles tells you at Fishdoc.  Radical approaches are possible where a seasoned veterinarian treats large koi, but you won't be injecting the Neon Tetras with antibiotics.
 
Newbies are often plagued by "fin-nipping," even in communities of mild-mannered tankmates. When I hear that, I often suspect fin rot. More advanced fishkeepers still may feel there must be some "physical" cause for fin deterioration — the betta has snagged its flowing fins on a rock, etc. I think it's just as likely that a rich load of organics dissolved in the water has encouraged minor ectoparasites, like Epistylis or skin flukes (Gyrodactylus), and that the irritation they cause has opened opportunities for bacteria and Saprolegnia. Since rich loading of dissolved organics encourages a dense population of flagellates and ciliates, some of which are opportunistic skin parasites, then the "stress and deteriorating water quality" that are so often invoked as "causes" of finrot, are in fact  predisposing factors after all.
 
 
Treatments: first step. Whatever medication route you decide on, the preliminary step is sanitation. You can do a few things to promote rapid healing yourself. For a start, you'll have better success with fungus cures if you precede them with a 50% water change. First siphon out all loose mulm and debris. Drag the edge of a credit card across the gravel, rolling over the uppermost layer: how much floc is accumulating there? Thie water change reduces the dissolved organic content of the water. You don't want the medication to expend itself oxidizing organics in the water. Follow up a few hours later, when the water is clear of floating debris, with a thorough backflushing of the filter. And slowly raise the temperature to 80°F — not enough to stress the fishes — because fungi grow more rapidly at cooler temperatures, which also slow fish metabolism.
 
Mouth fungus or finrot may not be directly communicable from one fish to another. Whether they are or not, many of the following treatments for fin rot are stressful to plants: can you isolate the afflicted fish in a Q tank?
 
Medications. You'll see anti-fungal patent medicines and herbal remedies locally and on-line touted to "promote healing." Read the ingredients of your patent anti-fungal medication with a critical eye. Anti-bacterial medications from the furan group or sulfa drugs — even the use of medicated feed — and now herbal tinctures are often recommended. When you read  "proper anti-bacterial fish medicines should not affect 'good' filter bacteria if applied in a proper dosage," your skepical alarms should ring: how is the medication achieving this selectiveness? is a "proper" dosage just a rhetorical disclaimer? I would look more to basic causes, first dealing with detectable ammonia and nitrite, increasing my sanitation regime, diluting and oxidizing dissolved organics, and then ridding the fishes' skin of minor ectoparasites.
 
Salt bath. Long-term salt baths and a gently warmer temperature are often all that is needed, if your alert eye has caught the incipient finrot when just the fraying tips of fins are showing white where they should be clear. The salt is particularly effective against Saprolegnia. In a planted aquarium, however, an effective concentration of salt will not be an option: isolate the affected individual in a hospital tank instead.
 
Dyes. Not all dyes are effective. Malachite green and formalin are often recommended as an anti-fungal remedy, though they can't be used with food fish. Now that malachite green has developed an exaggerated rep for toxicity it's sometimes presented as "Victoria Green". Acriflavine is sometimes recommended. Acriflavine is a powerfully staining and irritating dye, which you ordinarily buy pre-mixed. Acriflavine is not a fungicide. It will trash your plants, so if you use it, confine it to a plant-free Q tank.
 
Chloramine-T. Chloramine-T is a surface disinfectant used by veterinarians, with some use in koi ponds; it is effective against bacteria and protozoa. It is more effective and more toxic at lower pH. I thought that if your water were already chloraminated, you might simply try a series of daily water changes with un-dechlorinated water, but Anderson's comment (below) alerted me to look more carefully at the structure of Chloramine-T.  The Provet website suggests chloramine baths at concentrations well above what you'd normally find in your chloramine-treated tapwater.
 
Phenoxyethanol. Some patent medications are based on phenoxyethanol (C₈H₁₀O₂). A bath in phenoxyethanol is recommended for mouth rot by Drs Foster and Smith.
 
Naphthoquinones, which are natural phenolic compounds, are the active ingredients in Kordon's Rid-Fungus.
 
Potassium permanganate. If you act cautiously enough, potassium permanganate will oxidize residual organics, eliminate ectoparasites and attack fungi. It is the treatment to try when a salt bath is not halting fin erosion.