Pseudomonas Infection in Koi: Identification and Treatment

By KoiQuanta Editorial Team|Updated Mar 2026

Pseudomonas and Aeromonas are both common gram-negative bacterial koi pathogens that cause overlapping clinical signs. Most hobbyists have heard of Aeromonas; fewer know Pseudomonas by name - but if you've dealt with persistent fin rot or ulcers that didn't respond well to standard antibiotic treatment, Pseudomonas may have been the reason.

The distinction matters clinically because the antibiotic sensitivity profiles differ, and Pseudomonas is often more resistant to the compounds koi keepers typically reach for first.

TL;DR

Minimum course: 10–14 days of systemic antibiotic treatment, continued for 5–7 days after apparent clinical improvement.
If the fish "looks better" on day 7 of a 14-day course, finish the 14 days.
Post-treatment observation hold: 7–14 days with no relapse before discharge criteria can be met.
Clues that suggest Pseudomonas include primary fin erosion as the first sign, treatment failure on oxytetracycline after 5–7 days, and blue-green tinge or distinctive fruity odor to wound discharge (not always present).
Minimum 10–14 days of systemic antibiotic treatment.

What Pseudomonas Looks Like

Pseudomonas fluorescens and related species cause a range of presentations:

Fin rot and erosion: The most common early Pseudomonas presentation. Progressive fraying and erosion of fin tissue - starting at the fin tips and working toward the base. The eroded edges often appear ragged or whitish. This differs from mechanical fin damage (clean splits) and from Columnaris (faster progression, saddle-shaped body lesions).

Hemorrhagic septicemia: Redness at fin bases and throughout the skin, often described as "red veins" or bloody streaking in the fins. This presentation is also seen with Aeromonas and with koi pond water quality tracker problems (ammonia toxicity causes similar redness). Clinical context and treatment response help differentiate.

Ulcers: Pseudomonas causes ulcers, similar to Aeromonas. Pseudomonas ulcers tend to have more of a spreading, diffuse character compared to the more punched-out, defined margins typical of Aeromonas ulcers - though this distinction isn't reliable enough to diagnose from appearance alone.

Systemic septicemia: In immunocompromised fish or very fast-progressing cases, Pseudomonas can cause internal organ damage with generalized deterioration. This presentation overlaps completely with Aeromonas systemic infection.

Pseudomonas vs. Aeromonas: Key Differentiators

You cannot reliably distinguish these two pathogens from clinical appearance alone. The definitive answer comes from bacterial culture and sensitivity testing.

Clinical clues that suggest Pseudomonas over Aeromonas:

Primary fin erosion: Pseudomonas tends to cause fin rot as a primary presentation more commonly than Aeromonas, which more often starts with skin ulcers
Treatment failure on tetracyclines: Pseudomonas is often resistant to oxytetracycline and similar tetracycline-class antibiotics; Aeromonas is also increasingly resistant but historically more susceptible
Presence of blue-green or fruity-smelling wound discharge: Some Pseudomonas strains produce characteristic pigments (pyocyanin) and distinctive odor, though this isn't always present
Response to fluoroquinolones: Pseudomonas typically remains susceptible to enrofloxacin (a fluoroquinolone) longer than it does to tetracyclines

None of these is definitive. Culture and sensitivity testing is the only way to confirm Pseudomonas and determine which antibiotics will work.

For any bacterial infection that isn't responding to the first-line antibiotic after 5–7 days, culture and sensitivity testing through an aquatic veterinarian should be the next step.

Why Pseudomonas Is More Antibiotic-Resistant

Pseudomonas species have several intrinsic resistance mechanisms that make them harder to treat than many gram-negative bacteria:

Low outer membrane permeability: Many antibiotics can't penetrate the outer membrane of Pseudomonas effectively
Efflux pumps: Pseudomonas actively pumps certain antibiotics out of the cell
Biofilm formation: Pseudomonas forms protective biofilms that dramatically reduce antibiotic penetration to the bacteria within

This is why "the fish didn't respond to oxytetracycline" is often a Pseudomonas indicator. The resistance is built in, not just acquired through antibiotic overuse - though overuse does drive additional resistance development.

Treatment Protocol

First Response

If you suspect bacterial infection and can't wait for culture results:

Enrofloxacin (fluoroquinolone): Better Pseudomonas coverage than tetracyclines. Requires veterinary prescription in the US. Bath treatment or injectable (injectable has better tissue penetration for serious cases). 10–14 day minimum course.

Tobramycin or gentamicin (aminoglycosides): Generally effective against Pseudomonas where available and appropriate. Prescription required. These have nephrotoxicity concerns with prolonged use and are typically used under veterinary supervision.

Ciprofloxacin: Another fluoroquinolone with good Pseudomonas activity. Available for veterinary use.

What to avoid without a sensitivity result: Oxytetracycline, erythromycin, and penicillin-class antibiotics have poor or variable activity against Pseudomonas. Starting with these and not seeing response while the fish deteriorates is a common treatment failure pattern.

Wound Care

For visible fin rot or ulcers:

Daily topical treatment with povidone-iodine or chlorhexidine on affected tissue
For fin rot: debridement of visibly necrotic fin tissue to clean margins (this stops the tissue from providing a bacterial substrate)
Antibiotic wound paste applied to open wounds if available
0.3% salt as a supportive measure

Water Quality

Pseudomonas infections, like Aeromonas, are frequently triggered or worsened by water quality compromise. Elevated ammonia, nitrite, or chronically poor oxygen levels create the conditions where Pseudomonas can establish.

Ammonia: 0 mg/L
Nitrite: 0 mg/L
Temperature: 22–24°C during treatment (supports immune function; avoid extremes)
Dissolved oxygen: 7+ mg/L
0.3% salt throughout treatment

Isolation

Any fish with active Pseudomonas infection should be isolated. Pseudomonas is waterborne and infectious through skin wounds. A fish with active fin rot is shedding bacteria into the water.

Fin Rot Treatment Specifically

Fin rot is a common presentation that's often managed topically rather than systemically - sometimes successfully, sometimes not.

For minor fin tip erosion in an otherwise healthy fish:

Remove the fish from the pond
Trim damaged fin tissue to clean, healthy fin base with sterile scissors
Apply povidone-iodine to the cut edge
Improve water quality (fin rot is often a water quality disease)
Add 0.3% salt
Monitor daily

For fin rot that's progressing despite topical management, or that has reached the fin base:

Systemic antibiotic treatment is required
Culture and sensitivity testing is strongly recommended
Trimming alone won't stop a systemic bacterial infection

How Long Is the Treatment Course?

Minimum course: 10–14 days of systemic antibiotic treatment, continued for 5–7 days after apparent clinical improvement.

Don't shorten the course. Pseudomonas treatment failure from incomplete antibiotic courses is the leading cause of relapsing infections and resistance development. If the fish "looks better" on day 7 of a 14-day course, finish the 14 days.

Post-treatment observation hold: 7–14 days with no relapse before discharge criteria can be met.

FAQ

How do I know if my koi has Pseudomonas vs Aeromonas?

You can't reliably distinguish them from clinical appearance - both cause ulcers, hemorrhage, and fin rot. Clues that suggest Pseudomonas include primary fin erosion as the first sign, treatment failure on oxytetracycline after 5–7 days, and blue-green tinge or distinctive fruity odor to wound discharge (not always present). Definitive identification requires bacterial culture from an aquatic veterinarian, which also gives you a sensitivity result to guide antibiotic selection.

What antibiotic treats Pseudomonas in koi?

Fluoroquinolones (enrofloxacin, ciprofloxacin) are the most reliably effective class against Pseudomonas. Aminoglycosides (tobramycin, gentamicin) also have good Pseudomonas activity. Both require veterinary prescription in the US. Oxytetracycline (the most accessible over-the-counter option for koi bacterial infections) has poor activity against most Pseudomonas strains and should not be the first choice if Pseudomonas is suspected.

How long is the treatment course for Pseudomonas in koi?

Minimum 10–14 days of systemic antibiotic treatment. Pseudomonas infections that don't receive a full antibiotic course frequently relapse, and each relapse may involve a more resistant bacterial population than the last. Complete the full course even if clinical signs have resolved - resistance development in your pond's Pseudomonas population is a long-term management concern.

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Sources

Associated Koi Clubs of America (AKCA)
Koi Organisation International (KOI)
University of Florida IFAS Extension Aquaculture Program
Fish Vet Group
Water Quality Association