Important: this page is educational. Suspected KHV, rapid deaths, severe gill distress, or unexplained losses require professional fish-health help and laboratory testing. Do not sell, give away, or move exposed fish while a serious infectious disease is suspected.
Start with a completely separate system
A quarantine tank or vat should not share water, filtration, nets, bowls, hoses, or siphons with the main pond. The purpose is to observe a new koi without exposing the established collection while transport stress settles and health problems become easier to detect.
Quarantine goals
| Goal | What it prevents | Practical action |
|---|---|---|
| Observation | Adding a fish that is still stressed from transport or hiding disease signs. | Keep new koi in a separate system for at least 30 days; extend the period when illness, unstable water, or diagnostic uncertainty persists. Watch appetite, breathing, flashing, mucus, feces, skin, and fin edges. |
| Containment | Moving parasites, bacteria, or virus risk into the main pond. | Use separate nets, bowls, hoses, siphons, towels, and measuring tubs. Do not share wet tools between systems. |
| Stable recovery | Confusing shipping stress with pond disease. | Provide strong aeration, cover the tank, keep ammonia and nitrite at 0 ppm, and avoid repeated handling. |
| Decision point | Letting hope replace evidence. | Only move the koi when it eats normally, breathes normally, shows clean skin/fins, and water records have stayed stable. |
Minimum quarantine setup
| Item | Standard | Why it matters |
|---|---|---|
| Tank or vat | Large enough for calm swimming, covered, shaded, and protected from predators. | Jumping, overheating, and panic injuries are common quarantine failures. |
| Biofilter | Mature sponge filter, moving media, or seeded media from a known-safe system. | New quarantine tanks can produce ammonia spikes faster than owners expect. |
| Aeration | Air pump and diffuser running continuously; backup air is ideal. | Medication, warm water, and stressed fish all increase oxygen demand. |
| Testing | Ammonia, nitrite, pH, KH, nitrate, temperature, and dechlorinator for all new water. | Clear water can still be dangerous. Actual numbers prevent guesswork. |
| Dedicated tools | Separate net, bowl, hose, siphon, towel, and waste bucket. | Biosecurity fails most often through shared wet equipment. |
Do not add another fish after quarantine has started; mixing arrivals can introduce a new exposure and means the observation period should begin again. Do not use antibiotics or parasite treatments routinely without a diagnosis or professional guidance.
KHV: why koi keepers take quarantine seriously
Koi herpesvirus disease is a serious viral disease of koi and common carp. It can cause high mortality and may present with lethargy, loss of appetite, sunken eyes, pale or necrotic gills, skin changes, and sudden deaths. Signs can overlap with water-quality failure, parasites, and bacterial gill disease, so visual diagnosis is not enough.
Temperature matters because outbreaks are often associated with permissive water temperatures, commonly reported around the mid-teens to upper twenties Celsius range. A fish can look normal outside an active outbreak window, then become sick after stress or temperature change. That is one reason quarantine should be long enough to observe behavior over stable conditions rather than just a few days in a tub.
When to stop movement and seek testing
- More than one koi in quarantine becomes lethargic, stops eating, or breathes hard.
- Gills look pale, patchy, necrotic, or unusually mucus-covered.
- Deaths occur after recent purchase, transport, show attendance, or mixing of fish from different sources.
- Water tests are acceptable but fish continue to decline.
- You are thinking about moving fish, selling fish, or returning fish while the cause is unclear.
Safe entry decision
| Question | Move to main pond only if... | If not... |
|---|---|---|
| Water record | Ammonia and nitrite stayed at 0 ppm; pH and temperature were stable. | Extend quarantine and correct the system before judging the fish. |
| Behavior | The koi swims normally, eats strongly, and does not flash, gasp, isolate, or clamp fins. | Observe longer and investigate water, parasites, and gill condition. |
| Skin and fins | No ulcers, cotton growth, excess mucus, ragged fins, raised scales, or visible parasites. | Treat the confirmed problem in quarantine, not in the main pond by habit. |
| Source risk | You know the seller's quarantine practice, origin, and recent health history. | Be more conservative; mixed-source fish and recent show fish deserve extra caution. |