Koi are hardy, resilient, and often dramatic creatures—but when they get sick, they don’t come with little fish-sized instruction manuals. Suddenly you’re Googling at midnight, staring at bottles of green liquids, powdered chemicals, and medications with names that sound like wizard spells. “Formalin? Potassium permanganate? Dimilin? Am I healing my fish or summoning something?”
Don’t worry. With the right knowledge, medications become powerful tools—not mysteries. This guide will help you understand when to medicate, how to do it safely, and how NOT to accidentally turn your pond into a chemistry experiment gone wrong.
1. The Golden Rule: Diagnose Before You Dose
If koi keepers had a universal motto, it would be this:
Never medicate “just in case.”
Medicating blindly is like throwing darts in the dark—you might hit something, or you might poison the whole pond. Many koi illnesses show similar symptoms, and each medication treats something very specific.
Before medicating:
- Check water parameters (ammonia, nitrite, nitrate, pH, KH).
- Look for behavioral clues (flashing, gasping, isolation).
- Observe physical signs (spots, ulcers, clamped fins, frayed fins).
- When possible, get a scrape and scope from an experienced keeper or vet.
Fun fact: Up to 80% of koi “diseases” are actually caused by poor water quality—not actual pathogens. Test first, treat later.
2. When Medication Is Truly Needed
There are three main situations where medication becomes necessary:
1. Parasitic infections
Parasites like flukes, Costia, Trichodina, and Ich almost always require medicated treatment.
2. Bacterial infections
Ulcers, fin rot, and systemic infections may require topical treatments or antibacterial medications.
3. Fungal or secondary infections
Fluffy white patches, cottony areas, or infection on healing wounds can need antifungal support.
But again: identify what you’re treating before you treat it.
3. Treat in Quarantine Whenever Possible
Your pond is a giant ecosystem. Dropping harsh chemicals into the whole thing is like fumigating your entire house to get rid of one fruit fly.
Better option: Use a quarantine tank.
Advantages:
- Uses less medication (saves money).
- Protects your biofilter from damage.
- Makes observation easier.
- Prevents stressing healthy fish unnecessarily.
Some medications can wipe out beneficial bacteria—quarantine isolates the situation instead of nuking your pond.
4. The “Big Six” Koi Medications and What They Treat
There are many products, but most fall into a few key categories. Here’s a quick cheat sheet:
1. Praziquantel
Gentle and effective for flukes (both skin and gill).
2. Potassium Permanganate (PP)
A strong oxidizer that treats Costia, Trichodina, fungus, and organics.
Warning: PP will stain everything purple—including you. Always wear personal protective gear when handing Potassium Permanganate.
3. Formalin + Malachite Green
Effective for Ich, Costia, Chilodonella, fungus.
Warning: Never use in warm water or with salt above 0.1%.
4. Salt
Not glamorous, but excellent for general stress, early parasite stages, and nitrite issues.
5. Dimilin or Diflubenzuron
Targets anchor worms and fish lice.
6. Antibiotics (injectable or medicated food)
Reserved for serious bacterial infections or ulcers.
Pro tip: Antibiotics should NEVER be used randomly—they require diagnosis and a veterinary prescription.
5. Read the Label
Medications are not suggestions. They are chemistry. The dosage, timing, and safety notes on the label matter a LOT.
Before using any treatment, check:
- Water temperature requirements.
- Compatibility with salt levels.
- Required aeration (most need extra oxygen).
- If the medication harms plants or biofilters.
- If you need to remove carbon filters or UV clarifiers.
Pro tip: If the bottle says “turn off UV,” do it. UV can neutralize medications faster than a koi can spit out a pellet.
6. Measure, Don’t Guess
Guessing doses works only if you enjoy risk and chaos. Too little medication = ineffective. Too much = dead fish.
Always measure:
- Pond volume (calculate accurately—not “around 2,000 gallons”).
- Medication weight or liquid dosage.
- Salt levels (use a salinity meter).
Pro trick: When dosing large ponds, pre-dissolve medications in a bucket of pond water to get even distribution.
7. Aeration: Your Fish Need Extra Oxygen During Treatment
Many medications reduce available oxygen—or increase oxygen demand. This makes extra aeration absolutely essential.
If you medicate, you MUST:
- Run large air stones.
- Increase waterfall flow.
- Avoid shutting off your pump (unless the medication requires it).
If your koi gasps during treatment, stop immediately and oxygenate.
8. Never Mix Medications Unless You’re 100% Sure
Some medications are safe together. Many are not. A few combinations create koi chemistry nightmares.
Dangerous combos:
- Salt + formalin (above 0.1% salt)
- Multiple parasite meds at the same time
- Antibiotics + oxidizers
When in doubt, space treatments out by 48–72 hours.
9. Watch Your Fish During Treatment
Your koi will tell you if something is wrong—quickly.
Warning signs:
- Gasping at the surface
- Darting, jumping, or panic swimming
- Rolling sideways
- Clamped fins
- Sudden lethargy
If you notice these, stop treatment and oxygenate immediately.
10. Aftercare: The Forgotten Step
After treatment, your koi aren’t instantly “back to normal.” Their slime coat, immune system, and stress hormones all need time to recover.
Recommended aftercare:
- Add low-level salt (0.05–0.10%) to help healing.
- Perform partial water changes.
- Resume high aeration for 24–48 hours.
- Feed easily digestible foods (wheat germ or light diets).
- Observe for 5–7 days before assuming treatment is complete.
Bonus tip: UV clarifiers should stay off for 24 hours after treatment to avoid breaking down residual medication.
Medications are incredibly powerful tools in koi health—but they must be used with care, intention, and knowledge. Diagnose first, quarantine when possible, measure everything, and watch your koi closely.
Medications save koi lives when used correctly—and cause disasters when used blindly. When you medicate smart, you protect your fish, your pond, and your peace of mind.
Because healthy koi aren’t just beautiful—they make your entire pond feel alive.