Columnaris

Telling it from fungus

True fungus (Saprolegnia) is a cottony tuft that grows on already-damaged or dead tissue and collapses to slime out of water. Columnaris patches are flatter, often greyish-white to yellowish-brown, starting at the mouth, fin edges or gills and spreading outward; a pale band encircling the body — the classic "saddleback" — is characteristic, and an advanced saddle often develops a yellow-brown ulcer at its centre. Columnaris also moves faster than fungus and attacks the gills directly.

Treatments differ — antibacterial for columnaris, anti-oomycete for true fungus — so the distinction matters. When a "fungus" is spreading quickly, especially around the mouth or gills, or when you see a saddleback band, suspect columnaris and act accordingly.

• Greyish-white to yellow-brown flat patches, not fluffy tufts
• Classic pale "saddleback" band, often with a yellow-brown ulcer at its centre
• A yellow, mucus-like growth in and around the mouth
• Spreads fast and attacks the gills; doesn't collapse to slime out of water

The signs in detail

Columnaris lesions typically begin as dull, shine-less patches and progress to round or oval yellow-brown sores, sometimes ulcerated. The brownish colour often comes from mud and detritus trapped in the bacterial slime. On the gills the bacteria spread and erode tissue via protein- and cartilage-degrading enzymes, killing gill cells — which is why the gill form is so lethal: fish can die in numbers with little to see externally, showing mainly rapid breathing and distress. The infection can become internal; in one aquaculture study columnaris was isolated inside fish in around 40% of cases where it was found externally, sometimes with swelling of the posterior kidney.

• Skin: dull patches → round/oval yellow-brown, sometimes ulcerated sores
• Gills: spreading erosion via tissue-degrading enzymes — often the killer
• Mouth: yellowish, mucus-like bacterial growth
• Can go internal — external infection isn't the whole picture

The bacterium and how it's confirmed

Flavobacterium columnare has been renamed repeatedly — Bacillus, then Flexibacter, then Cytophaga, now Flavobacterium — which is why older references can be confusing. A long, thin, gliding bacterium, it is present in most, if not all, aquaculture environments, harmless to a healthy fish until stress and deteriorating conditions let it in.

A wet-mount scrape of an affected area gives a presumptive diagnosis under the microscope: the long, slender rods (roughly 5–10 µm, some sources report up to 30 µm) flex and glide across the slide using a characteristic flexing/gliding motility, and at the lesion margins aggregate into loose columns or "haystacks" that give the disease its name. That column formation is the key microscopic tell — Saprolegnia shows branching fungal hyphae with no motility; Epistylis shows stalked colonial ciliates; Costia (Ichthyobodo) shows small, fast-moving flagellates. On low-nutrient agar the colonies form characteristic rhizoid or "haystack" patterns. Definitive identification uses culture on specialised low-nutrient media (Cytophaga agar or similar) or molecular methods such as PCR — the route taken in aquaculture diagnostics and by aquatic pathology labs.

If external signs are ambiguous — or a fish is dying with only respiratory distress and you need to rule in or out gill columnaris — a fresh gill clip mounted wet is worth the effort. You're looking for that same gliding-rod picture, distinct from gill flukes (visible on a scrape as flat, muscular flatworms with opisthaptor hooks under low-to-medium magnification) or Ichthyophthirius trophonts (round, ciliated, with a horseshoe nucleus visible at 100×). When you cannot resolve the differential at home and the fish's life is genuinely in the balance, a wet-prep submitted to an aquatic pathology lab or reviewed by an aquatic vet via photo/video is the right call — OTC shotgunning is not.

Why it takes hold — and the temperature trap

Columnaris is a stress disease. The conditions that favour it are familiar: low oxygen, high ammonia or nitrite, crowding, rough handling, mechanical or cage injury, and warm water in the upper part of a fish's preferred range. In farmed catfish it commonly appears at roughly 77–90 °F, across spring, summer and autumn. That temperature link marks the single most important difference from ich: warmth accelerates columnaris. The reflex many keepers have — raising the temperature to push a parasite through its life cycle — is exactly wrong here. If your species tolerate it, holding the water at the cooler end buys time; the reverse makes things worse.

• Triggers: low oxygen, high ammonia/nitrite, crowding, handling injury, stress
• Common at warm temperatures (≈77–90 °F)
• Warmth speeds it — the opposite of ich
• If species allow, cooler water can slow it and buy time

The treatment toolkit — and its tradeoffs

Columnaris is bacterial and fast — treatment works on two fronts, and the right one depends on whether the infection is external, internal, or both. External infections are addressed in the water column with appropriate licensed agents: potassium permanganate is a traditional pond choice; salt and other antibacterial agents are used in tanks. Internal or systemic infection calls for an antibiotic, usually medicated feed, which is why catching it early — before a sick fish stops eating — matters so much.

Every agent has a cost, and the advanced keeper needs to hold those tradeoffs explicitly. Salt at therapeutic levels harms scaleless fish (loaches, many catfish, eel species, some plecos) — dip concentrations that a livebearers tolerates can stress or kill a kuhli loach. Many tetras, particularly small characins and pencilfish, are similarly sensitive. Copper-based treatments, if considered, will kill most invertebrates — shrimp, snails, and any other soft-bodied invert — often irreversibly for snails even after copper is removed, and copper also stresses scaleless fish. Formalin, if used at all, depresses dissolved oxygen significantly; increase surface agitation and monitor DO closely when using it in a closed system. Many antibiotics — particularly those that reach the water column — crash nitrifying bacteria; running a temporary hospital tank with pre-cycled media is far preferable to medicating a display tank. In planted tanks, some dye-based bacteriostats (e.g. acriflavine) inhibit plant growth and may discolour substrate.

Antibiotic selection matters. F. columnare has demonstrated acquired resistance, and resistance patterns vary by region and strain. Using the wrong antibiotic, or using one at sub-therapeutic opportunity (stopping too early, inconsistent medicated feed uptake), selects for resistant populations. If a case is not responding to a first agent and you can access diagnostic support, sensitivity testing rather than escalating empirically is sound medicine. Specific agents, doses and schedules belong to the cited sources or an aquatic veterinarian — this is not the place for a protocol.

• External: licensed water-column agents (e.g. potassium permanganate, salt) — salt harms scaleless fish and sensitive tetras
• Internal/systemic: an antibiotic, often medicated feed — treat before fish stop eating
• Copper: lethal to invertebrates; avoid in shrimp or snail tanks; stresses scaleless fish
• Formalin: drops dissolved oxygen — add aeration; avoid with already-hypoxic fish
• Antibiotics in the main tank: crash the biological filter — use a hospital tank where possible
• Planted tanks: dye-based bacteriostats may inhibit plant growth; move fish if possible
• Resistance is real — don't shotgun OTC meds sequentially; escalate to diagnostic support if no response

Biosecurity, quarantine, and treating the system

F. columnare is waterborne and moves fast. It can be introduced on fish, on plants, even on wet equipment shared between tanks. A proper 4–6 week quarantine — not a three-day "observation" period — matters here: fish shed the bacterium asymptomatically under low stress and break with disease only when stress rises in the display. A hospital or quarantine tank also allows targeted treatment without medicating the display's biofilter, invertebrates, or plants.

If one fish breaks with columnaris in a display tank, assume the entire system is exposed. The question is whether to treat the tank or isolate. If the display holds sensitive species (scaleless fish, shrimp, snails), treating in place is often worse than removing the sick individual and treating it separately. If the outbreak involves multiple fish or is spreading quickly, a whole-tank approach with an appropriate agent may be necessary — but remove invertebrates and switch to a hospital setup where you can. Shared equipment (nets, siphons, buckets) should be disinfected between tanks, particularly if you're running a fish room. The bacterium does not survive indefinitely in dry conditions, but a damp net is a vector.

• Quarantine 4–6 weeks, not days — fish shed the bacterium asymptomatically under low stress
• Treat isolated fish in a hospital tank wherever possible — protects the biofilter and invertebrates
• Multi-fish outbreak: consider a whole-tank approach, but remove invertebrates first
• Disinfect all shared wet equipment between tanks to stop cross-contamination

Prognosis and when to escalate

Caught early — a single saddle lesion, a fish still eating, good water quality — columnaris is winnable. Caught late, particularly in the gill form, it is often not. A fish in acute respiratory distress with gill haemorrhage and no appetite, found on a morning check, may not survive even correct immediate treatment; the tissue damage is simply too extensive. Honest assessment here saves time and avoids prolonged suffering: if a fish is in extremis, escalating to a hospital tank and administering whatever treatment you have is worth attempting, but the outcome is uncertain.

Escalate to an aquatic veterinarian or diagnostic lab when: a case is not responding after a full course of the first agent; you are working with a valuable fish or a breeding group; external signs don't match this differential cleanly and you need a confirmed diagnosis; or you have had repeated episodes in the same system and need sensitivity data to choose an effective antibiotic. A wet-prep or tissue sample submitted to an aquatic pathology lab can deliver a genus-level or species-level identification and antibiogram in days — far more targeted than empirical escalation.

• Early (single lesion, eating, good water): genuinely winnable with correct treatment
• Late gill form with respiratory distress and no appetite: prognosis is poor — act fast but be honest
• Escalate to a vet or lab if: no response to first agent; repeated episodes; ambiguous diagnosis; valuable stock
• An antibiogram tells you which antibiotic will actually work — far better than sequential OTC guessing