Acriflavine Treatment for Koi: Fungal and Bacterial Protocol

By KoiQuanta Editorial Team|Updated Mar 2026

Acriflavine is a photoactive compound -- UV exposure breaks it down within hours in ponds. This is the single most important practical consideration when using acriflavine as a pond treatment. A pond treatment in direct sunlight loses considerable potency within 4-8 hours, which effectively makes outdoor pond treatment with acriflavine unreliable unless you're treating at dusk or in a shaded area.

Bath treatments in a koi quarantine program tank -- away from direct UV exposure -- are more reliable and allow you to control the treatment concentration accurately.

TL;DR

A pond treatment in direct sunlight loses considerable potency within 4-8 hours, which effectively makes outdoor pond treatment with acriflavine unreliable unless you're treating at dusk or in a shaded area.
In a quarantine tank without UV exposure, acriflavine maintains effectiveness for 24-48 hours before significant degradation.
In an outdoor pond in direct sunlight, potency drops within 4-8 hours.
A 30-minute bath at 10 mg/L or a 24-48 hour treatment at 5 mg/L are standard approaches.
For extended quarantine tank treatment at 2-5 mg/L, redose daily to compensate for degradation.
For topical wound treatment, apply a more concentrated solution (0.1-0.2%) directly to the wound site on a briefly sedated fish.
In outdoor ponds in direct sunlight, potency drops meaningfully within 4-8 hours.

What Acriflavine Is and What It Treats

Acriflavine is an acridine dye antiseptic that was used extensively in wound treatment and antimicrobial applications before modern antibiotics. In fish keeping, it remains a useful topical antiseptic for:

Superficial bacterial infections: Surface-level bacterial lesions, very early fin rot before significant erosion has occurred, and bacterial film infections on skin. It's effective against a range of gram-positive and gram-negative bacteria at bath concentrations.

Fungal infections (Saprolegnia): Acriflavine has antifungal activity against the water mold fungal infections common in koi. It's less consistently effective than methylene blue for established Saprolegnia, but useful for early-stage or preventive application.

Wound treatment: Applied as a dilute bath or topically at wound sites to prevent secondary infection during healing.

Egg treatment: Used in spawning operations at very low concentrations to prevent fungal contamination of eggs during incubation.

What acriflavine does NOT effectively treat:

Established internal bacterial infections (septicemia, deep ulcers)
Parasitic infections (no antiparasitic activity)
Viral disease

For established bacterial infections, appropriate antibiotics are necessary. Acriflavine is a surface antiseptic, not a systemic antibiotic. For the full bacterial treatment framework, the koi bacterial infection treatment guide covers appropriate antibiotic protocols.

Dosing and Treatment Protocol

Bath treatment concentration: 5-10 mg/L for fish treatment baths. This is a relatively low concentration -- acriflavine is potent and should be used conservatively.

For commercial acriflavine solutions (typically sold at various concentrations -- check your product label):

10 mg/L treatment: Calculate mL of solution needed based on active ingredient percentage and tank volume

Duration: 30 minutes for a short bath treatment at 10 mg/L, or 24-48 hours at 5 mg/L in a treatment tank for extended exposure.

Tank treatment concentration for extended exposure: 2-5 mg/L maintained for up to 5 days. This range is generally well-tolerated by koi.

For wound topical application: A more concentrated solution (0.1-0.2%) can be applied with a cotton swab to wound surfaces on a briefly sedated fish. This isn't a pond treatment -- it's a local application to a specific wound site.

How Long Does Acriflavine Treatment Last?

In a quarantine tank without UV exposure, acriflavine maintains effectiveness for 24-48 hours before significant degradation. In an outdoor pond in direct sunlight, potency drops within 4-8 hours.

This UV degradation means:

Outdoor pond treatment: Add at dusk, allow the full overnight period for effectiveness, accept reduced potency during daylight. This is workable for overnight bath-style treatment but not for multi-day pond treatment.
Indoor quarantine tank treatment: Dose once daily to maintain effective concentration, or twice daily if testing confirms rapid degradation in your specific tank setup.
UV sterilizer: Turn off any UV sterilizer during acriflavine treatment. The sterilizer will degrade acriflavine passing through it, reducing treatment effectiveness.

Is Acriflavine Safe for Koi?

At recommended treatment concentrations, acriflavine is generally well-tolerated by koi. The main safety considerations:

Biofilter impact: Like methylene blue, acriflavine can inhibit nitrifying bacteria in your biological filter. Monitor ammonia and nitrite during and after treatment in tanks with established biofilters.

Photosensitivity: The same photosensitivity that causes UV degradation means fish that are treated with acriflavine and then exposed to bright sunlight may show increased sensitivity. Keep treated fish out of direct intense sunlight during treatment.

Extended use: Prolonged exposure at higher concentrations can cause irritation. Stick to recommended concentrations and durations.

For the quarantine medications context including how acriflavine fits alongside other quarantine-phase treatments, see the koi quarantine medications overview. For fungal infection treatment specifically, the koi fungal infection treatment guide covers the full treatment approach including when acriflavine is appropriate.

When to Choose Acriflavine vs. Other Treatments

Choose acriflavine when:

Treating superficial bacterial or fungal infections in a quarantine tank setting where UV is controlled
Applying topically to wounds on sedated fish as a surface antiseptic
Using in a spawning operation for egg protection

Choose methylene blue instead when:

The fish also shows signs of oxygen distress or has gill damage
You specifically want the oxygen transport support benefit

Choose salt instead when:

You need a low-impact, widely compatible treatment for osmotic support and mild antisepsis
You're treating in a planted system where dye staining is a concern

Choose antibiotics instead when:

Bacterial infection is systemic (ulcers with necrotic tissue, internal infection signs, septicemia)
Fin rot is progressing past very early stages

Frequently Asked Questions

How do I use acriflavine on koi?

For a bath treatment: dose at 5-10 mg/L in a quarantine tank. Turn off UV sterilizers before dosing. A 30-minute bath at 10 mg/L or a 24-48 hour treatment at 5 mg/L are standard approaches. For extended quarantine tank treatment at 2-5 mg/L, redose daily to compensate for degradation. For topical wound treatment, apply a more concentrated solution (0.1-0.2%) directly to the wound site on a briefly sedated fish. Avoid outdoor pond treatment in direct sunlight as UV degrades acriflavine rapidly.

How long does acriflavine treatment last?

In a shaded quarantine tank, acriflavine retains significant activity for 24-48 hours. In outdoor ponds in direct sunlight, potency drops meaningfully within 4-8 hours. This UV sensitivity makes outdoor pond treatment less reliable than quarantine tank treatment. For quarantine tank use at treatment concentrations, redose daily. Turn off UV sterilizers during treatment as they rapidly degrade acriflavine passing through them. Treat at dusk if pond treatment is necessary and accept the overnight window as the primary treatment period.

Is acriflavine safe for koi?

Yes at recommended concentrations. Acriflavine is generally well-tolerated by koi at 2-10 mg/L for standard treatment durations. The main practical concern is biofilter impact -- like most antiseptic dyes, it inhibits nitrifying bacteria, so monitor ammonia and nitrite during and after treatment in tanks with established biological filtration. Keep treated fish out of direct intense sunlight during treatment due to photosensitivity. Do not use beyond recommended concentrations or durations, and don't combine with other dye medications without veterinary guidance.

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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