In the koi world, “quarantine tank” doesn’t sound glamorous. It sounds like paperwork and waiting rooms. But for your pond, a quarantine tank is more like a VIP backstage area—where every new koi checks in before joining the main show.
Done right, a quarantine tank protects your pond from parasites, disease, and heartbreak. Done wrong (or skipped completely), one sick newcomer can turn your peaceful oasis into a drama-filled disaster. Let’s walk through how to set up a quarantine tank that keeps your koi—and your sanity—safe.
1. Why You Need a Quarantine Tank (Yes, You)
Even if you buy from the best breeder on the planet, new koi are stressed from transport, new water, and new surroundings. Stress = weakened immune system = perfect opportunity for parasites and bacteria to party.
Quarantine gives you time to:
- Observe new fish for hidden illness.
- Treat parasites or infections without medicating your entire pond.
- Acclimate koi to your water parameters and temperature.
Think of it like dating: you don’t marry someone after the first coffee. You get to know them first. Your koi deserve the same courtesy.
2. Choosing the Right Tank or Tub
You don’t need a fancy glass aquarium. Koi are not picky about aesthetics—just water quality and space.
Good options:
- Food-grade plastic stock tanks.
- Intex-style framed pools.
- Large, smooth-sided tubs (no sharp edges).
Size guidelines:
- Small koi (6–10 inches): 150–300 gallons.
- Larger koi (10–20+ inches): 300–500 gallons or more.
More water = more stability and forgiveness. If you’re on the fence about size, always go bigger. (Koi rule of life.)
3. Filtration: Your Mini-Pond’s Life Support
A quarantine tank isn’t just a bucket of water—it’s a miniature pond system. That means you need filtration and aeration, not just a hose and hope.
Filtration must do two things:
- Mechanical filtration: Removes poop, uneaten food, and debris.
- Biological filtration: Houses beneficial bacteria that convert toxic ammonia → nitrite → nitrate.
Popular choices:
- Pressurized pond filters with built-in UV.
- Simple barrel filters with media (like Japanese mat or moving bed media).
- Oversized canister filters (for temporary setups).
Important: A brand-new filter with no bacteria is like a brand-new office with no employees—nothing gets processed. Ideally, “seed” your quarantine filter with media from your main pond filter so it starts with good bacteria already working.
4. Aeration: Keep the Bubbles Going
New koi + stress + possible medications = higher oxygen demand. Quarantine tanks often have higher fish density than your pond, so oxygen can drop fast.
Must-haves:
- Air pump with air stones or diffusers.
- Surface movement (from an airstone, return line, or small waterfall).
Rule of thumb: If the water surface looks like a still mirror, add more air. Your koi—and your biofilter bacteria—are both oxygen addicts.
5. Heating and Temperature Control
Warm, stable water speeds up parasite life cycles, boosts immune response, and makes observation easier. Many koi keepers aim for 74–78°F (23–25°C) in quarantine.
To do this, you’ll want:
- A submersible or inline heater rated for the volume of the tank.
- A thermometer you actually check.
- Some form of insulation if the tank is outdoors (foam board, cover, or even an insulated shed).
No wild temperature swings. Koi like consistency more than surprise parties.
6. Water Testing: Your Quarantine Dashboard
In a small system, water quality can shift quickly. You are the pond’s life support technician now—congrats!
Essential test kit parameters:
- Ammonia
- Nitrite
- Nitrate
- pH
- KH (carbonate hardness) if possible
Test daily at first, then at least several times per week. If ammonia or nitrite show above zero, you’ll need water changes and maybe some bottled bacteria to help the filter catch up.
7. Setting Up the Tank: Step-by-Step
- Place the tank: Choose a shaded, level area away from heavy traffic or predators.
- Fill with dechlorinated water: If using tap water, add a dechlorinator that handles both chlorine and chloramine.
- Install filtration and aeration: Get everything running before the fish arrive. Let it run for at least a day if you can.
- Check temperature and pH: Aim to match (as closely as possible) the source water where your koi came from or where they’ll eventually live.
- Cover the tank: Use a net or rigid cover—new koi are Olympic jumpers.
Now you’ve got a functional little koi safehouse.
8. Moving Koi into Quarantine
When your new koi arrive, don’t just dump them in like goldfish at a carnival. They’ve been through a journey—bags, boxes, trucks, maybe planes.
Acclimation steps:
- Float the unopened bag in the quarantine tank for 15–20 minutes to equalize temperature.
- Open the bag and gradually add small amounts of tank water every 5 minutes for 20–30 minutes.
- Use a net to gently transfer koi into the tank (do not pour bag water into your system).
- Leave the lights low or keep things calm for a few hours to let them settle.
No food for the first 12–24 hours—let their systems relax.
9. How Long Should Quarantine Last?
The magic number most experienced keepers use is 3–4 weeks. That window gives time for:
- Hidden parasites to show themselves.
- Wounds or infections to appear (if they’re brewing).
- You to treat any issues without risking your main pond.
During this time, watch daily for:
- Appetite (are they eating eagerly?).
- Behavior (any flashing, gasping, isolation?).
- Body condition (spots, ulcers, fin damage, cloudy eyes?).
Pro tip: Keep a little “logbook”—notes on behavior, water tests, treatments. It’ll save your brain later.
10. Medications and Preventive Treatments
Some koi keepers proactively treat new arrivals for parasites; others prefer a “wait and see” approach and only treat if symptoms appear. Whichever camp you’re in, the quarantine tank is where all treatments should happen—not the main pond.
Common tools:
- Non-iodized salt (for stress relief and mild parasite support).
- Approved parasite treatments (for flukes, Costia, Trichodina, etc.).
- Topical treatments (only for ulcers and only if you know what you’re doing).
If you’re unsure, consult a koi-savvy vet or experienced hobbyist before throwing in chemicals “just because.” Guess-treating can do more harm than good.
11. Ready for the Main Pond? Final Checks
By the end of quarantine, your koi should be:
- Active and curious.
- Eating with enthusiasm.
- Free of spots, sores, frayed fins, and odd behavior.
Before moving them to the main pond:
- Make sure pond and quarantine water temperatures are within a few degrees of each other.
- Acclimate again (float in a tub or bag, mix pond water slowly).
- Transfer koi with a net—no quarantine water into the pond.
Congrats—your new koi are now cleared for full pond citizenship.
A quarantine tank may not be the prettiest part of koi keeping, but it’s one of the most important. It’s your first line of defense, your observation zone, and your problem-solving space—all rolled into one.
Set up a properly sized, filtered, aerated, and heated quarantine tank; test your water; watch your fish; and give them a few weeks to prove they’re healthy. Your reward? A safer pond, stronger koi, and way fewer midnight panic searches about “weird white spots on fish.”
Your koi may never thank you out loud—but they’ll show it in clear eyes, strong fins, and years of calm, graceful swimming in your main pond.