Know What You’re Getting Into
Getting koi or goldfish from someone else’s pond can be a win-win — you help rehome fish, save money, and give them a new home. But it’s important to be aware of the risks and what to watch for before you say “yes” to that bucket of fish.
Having fish is fun and relaxing. Did you know they LOVE watermelon?
First make sure your pond is large enough for more fish
Before you even consider adding more fish, check your pond’s capacity.
Rule of thumb: Every 10 inches of koi needs about 100 gallons of water (and remember — they grow!).
Use our Pond Gallon Calculator before adding fish. You’ll need to know the longest length of the pond and the widest width, as well as the depth. Overcrowding can cause poor water quality, stress, and even illness.
Look for Healthy Fish
Most look fish look fine on first glance, but sometimes they can have a parasite, fish lice, dropsy Ich or fin rot. You don’t want to spread disease to your entire pond!
Avoid fish with:
White or gray fuzzy spots – could be fungus or Ich.
Pinecone-like scales – a sign of dropsy. It looks like the scales are sticking out.
Ulcers or sores – possible infection or injury.
Clamped fins (held tight against the body) – could indicate flukes or fish lice.
Lethargy or isolation – healthy fish should be curious and swim with the group.
- Scraping – Watch for fish that are scraping their bodies along pots or rocks. This can be a sign of fish lice.
If the fish keeps her fins clamped, there is an issue. (Yes, this is a female fish!)
This white fish is very red where it shouldn’t be. This is likely an inner bacteria infection.
You might think you wouldn’t know what to look for. But once you are looking at them, you can actually tell when something is not right.
Signs of a healthy fish:
Active swimming and normal interaction with other fish.
Clear eyes and smooth, clean skin.
No visible spots, lesions, or ragged fins
Some people say not to name the fish….here are a handful of ours….
Be Ready for the Big Transfer
Fish can be very resilient, but if you don’t transfer them carefully, they could die.
- Easiest to transport them in a cooler. It has a lid too!
- Make sure that the pond they are coming from is being pumped down the day you come for the fish. They are near impossible to catch in 3 feet of water. (Fast little suckers!) Chasing them around for a while with a net stresses them out too.
- Take a net, or a Fish Sock if they are over 12 inches long. When they are larger it is hard on them to be in a normal net.
- If they will be transferred more than 30 minutes, take the time to get a Portable Aerator (easy to get off Amazon)! They give just enough air bubbles to keep them from being so stressed out on the drive.
Once the fish are moved from the pond into a holding tank or tub, it is easier to inspect them.
Once You are Home
- Place the cooler near the pond, but in a shaded spot
- Open the lid, and don’t walk away from the fish. They can jump out.
- Splash a bit of pond water into the cooler. Start with a 1/2 cup or a cup. You are doing this for 2 reasons.
- #1: the temperature of the pond and the water in the cooler needs to be within 5 degrees.
- #2: You need to match pH between the old pond and yours. Sudden changes can stress or kill fish.
- Use a water conditioner like Stress Coat to help reduce stress. Add to the water in the cooler (follow directions on the bottle). This is not 100% necessary but it is a good idea!
- Continue to add small amount of pond water into the cooler every 10 to 15 minutes for at least 30 minutes.
- After 30 minutes put your hand in the pond water and then feel the cooler water. If it feels the same, gentle put the cooler in the pond and let them swim out. If you can feel a difference in the temperature, give it more time splashing in pond water.
- You may see on line that you should Quarantine new fish for a couple of weeks in a separate tank. This is ideal, though not always practical. Most of us don’t have a separate fish set up to do this. We have seen many fish transfers that go just fine without it! If you have show koi that you have spent thousands of dollars on, you should have a quarantine tank ready to go anyway!
Moving fish in a cooler works really well. Just make sure you have 2 people to lift this into the car. Don’t forget to drive gently!
Adding Fish to a Brand-New Pond
If your pond has never had fish before, start small — just 2–4 fish at first.
The pond’s ecosystem needs time to “cycle” and build up beneficial bacteria to handle fish waste.
After a couple of weeks, you can add 3–4 more fish at a time, following the same directions above, until you reach your safe stocking level. If you’re new to koi keeping, start with inexpensive fish while you get the hang of things.
Why does it matter how many koi or goldfish are in the pond?
You may have seen the rule of thumb above that says you need 100 gallons of water for every 10 inches of koi. But WHY? And what if you have say 10 small fish that are 4 inches long? Most of us count how many fish we have not the inches!
True. But if you have a pond with fish you have to realize that those fish will GROW! If you have 10 little fish about 4 inches long you may only have 40 inches of fish, but they will grow to be 10 inches, 12 inches and larger. You might end up with 1000+ inches of fish!
These fish POOP! They PEE! In turn this waste is adding nutrients to the pond which will do a couple of things. It will feed algae. So you may have more algae than you want if the pond ecosystem cannot handle it. (I hop you have a BioFall and LOTS of PLANTS!
If the pond is overloaded you can suddenly have an ammonia spike. Ammonia is measure in parts per million (ppm) and it is supposed to be at zero. If the ecosystem can’t process the waste through bacteria, plants, BioFalls, stream and waterfall drops with plenty of oxygen and circulation, a sudden spike in ammonia can kill all your fish at once!
Koi or Goldfish?
When you put in your pond size in to our Pond Gallon Calculator , scroll down a little and it will tell you how many gallons of water you have an how many koi or goldfish your pond can safely sustain. Did you know you can have more goldfish in a pond than koi? You can! They are more tolerant.
Did you know the fish will be find if you put goldfish and koi in the same pond? They will! However, you will get some babies that are part goldfish and koi that may come out a plain dark grey or black.
Too Many Fish? We Can Help.
If you find yourself with more fish than your pond can handle, give us a call — we can help rehome the extras. The best time to have some fish rehomed is during a pond cleanout. It’s always better to think ahead when adding new fish.
Will My Pond Be Overrun With Babies?
Koi do spawn (usually May–June in Colorado), but they also eat many of their own eggs. In our pond, we’ve never had more than 5 ot 8 survive in a year.
Water Testing — Optional
The API Pond Test Kit has test options for pH, ammonia, nitrites, and nitrates. Testing can help you spot issues early, but testing often is really overkill. I honestly don’t recommend testing for pH. It can swing within a day and if you choose to use additives to bring up or down, you can cause even more problems. I recommend testing if the fish are acting sick just to verify if pH or ammonia is an issue. At that point a partial water change is the first thing I would do.
Once a pond’s ecosystem is in balance, most pond owners rarely test the water.
Adding fish is one of the most fun parts of pond keeping — but do it slowly, choose healthy fish, and make the transition gentle. Watching your fish swim and interact is as relaxing as sitting by a crackling fire.
These are a bunch of fish fry. NO! Not a fish FRY! Baby fish are called “Fry”!
Kristi's Tips
Don’t feed fish that are brand new to the pond right away, especially if they are the only fish in the pond. They may be shy and will not even come up for food.
Don’t be surprised if new fish hide for several days after the big move.
When you receive fish from another pond it is always a risk. We have done it many times, and you can always get koi from a reputable dealer like Next Day Koi or the fabulous fish guy we know in Parker. It will be more expensive if you purchase them, so do what works for you.
* This blog was NOT written by AI. This was written by me. I am not a professional writer, I am a pond person. Therefore, if you see a typo, please forgive me!
*Affiliate Disclosure: Just so you know, some of the links in this post are affiliate links. This means that if you click through and make a purchase, I may earn a small commission at no extra cost to you. It helps me keep sharing pond tips, stories, and resources — and I truly appreciate your support!