Sick Koi

Sick Koi

Sick Koi but good water parameters in outdoor Koi Pond??


I’m currently babysitting my parents’ outdoor koi pond and will be doing so for about 2 weeks. It’s quite a large pond (10,000 gallons and spans half of our backyard). We have several large koi (length of adult forearm) and several smaller goldfish (length of adult palm) and based on the number of fish, the pond may be overstocked. The outdoor temperature in Vancouver has been quite variable this summer (16 – 32 degrees Celcius. Low-mid 20’s this week but the temperature was up to 30 degrees over the last 2 weeks). For some reason the pond is losing about 4 inches of water each day so we refill the pond with a bit of fresh water each day (like a 5% daily partial water change!). My parents have had this pond for about 5 years.

At first glace, the fish look relatively healthy (full scales/fins) but there are always a few fish in the pond that have finrot, sores on the body or strange growths. Some fish have had raised scales, pineconing and died not having released their eggs. The fish have been exposed to salt and antibiotics many times. Sometimes the treatment works, but on average about 1 fish dies each month and my parents replace them. At the moment, one very large fish currently has sores on one side, some raised scales and prefers to swim at the bottom of the pond and doesn’t seem interested in food. Another strange observation: when I fed the fish this morning, I noticed that 3 smaller fish appeared to be chewing/biting another small fish of similar size in a corner of the pond. When I threw food at them, all of them swam away. What does this mean??

Seeing that there are always ill fish, I assumed that there must be problems with the water quality. However, when I tested the water parameters of a sample of pond water today, they seemed ok:
Ammonia: 0-0.1
Nitrite: 0-0.1
Nitrate: 5-10
pH: 7.2-7.4
KH: 10
GH: 20
Phosphate: 1

I came across your site and thought that the garlic may help. My parents usually feed Hikari pellets twice a day, so over the last 2 days I have been feeding a mix of presoaked pellets and cooked peas but I think only the smaller fish were able to see the crumbled peas because the large koi swam right through the small bits.

I have attached some images of the pond. Please let me know if you have any ideas




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  1. emme November 2, 2017 at 3:04 pm

    Hi Venus,

    My parents would like to unplug 2 submerged pumps. They are asking if we should cover the top of that area of the pond with anything to keep it warmer?

  2. Venus November 2, 2017 at 6:59 am

    Hi Emme,

    A very good thought indeed, but I wouldn’t worry too much. With a pond your size, there’s bound to be an area that the pumps aren’t reaching; some quiet spots. The area where the water fall is located for example. To be sure, turn off the pump on the far end. I don’t have a lot of experience with the extreme cold you experience. Here in Missouri we have relatively mild winters by comparison

    I’m sure the water conditions won’t deteriorate if only one pump is turned off. The fish won’t be using much oxygen over the winter anyway

  3. emme November 1, 2017 at 11:41 pm

    Hi Venus,

    Outdoor temperatures are now consistently below 10 degrees Celsius 24/7. The pond guy says that since we have 5 pumps submerged at the bottom of the pond now, the base of the pond may be a lot colder than previous years and may be even too cold for the fish, as the pond bottom would have been warmer for the fish during previous Winters when there were no pumps there. What do you think about his comment, Venus? Do we need heaters? We’ve never had any installed…

  4. emme October 17, 2017 at 12:06 am

    Sounds good. Will keep monitoring the water. Many thanks to you, Venus 😀

  5. Venus October 15, 2017 at 8:07 am

    That is a wow, and a big one at that, considering everything you guys have gone through. This is the way it’s supposed to be with big ponds; easy

    If the fish aren’t feeding, they’re in a dormant state. If the fish are in a dormant state, so are the friendly bugs. As the weather warms, and the fish start feeding again, so will the friendly bugs. No, there shouldn’t be any spikes in the cycle. This only happens when food is added to the pond that the fish don’t eat. The fish will let you know if they’re wanting food by swimming to the surface, so play it by ear

    You did it

  6. emme October 15, 2017 at 2:41 am

    Hi Venus,
    Wow, since the pond leak has been fixed, I’ve just been doing weekly water tests and it looks like the good water parameters have been consistent over the last 2 weeks. I didn’t add anything to the pond last week and when I measured today these were the readings: Ammonia=0.1, nitrite =0.1, nitrate=10, KH=130, GH=240. I might add more epsom to increase GH again but I’m quite pleased by how low maintenance the entire pond has been. I’m thinking it may also have to do with my dad having stopped feeding over the last week (outdoor temperatures are 6-12 degrees C thes days). This makes me wonder though, what happens to the good bugs in the pond when the temperatures are so low? Do they hibernate like the bad bugs? Would water parameters spike once it’s warmer after Winter?

  7. Venus October 5, 2017 at 10:45 am

    The pond pumps produce action at the surface, but the bottom remains calm, which is perfect for Koi, especially if the water is oxygenated on the bottom, and it is. There’s a very good chance the increased action will keep the areas around the pumps from freezing. They say fast moving water doesn’t freeze, but who are they? It depends on how cold and for how long the cold spell lasts. Fingers crossed. I think it will be fine

    It’s fine for your dad to feed toasted wheat germ now, but this mostly depends on the water temps. Large bodies of water take a lot of time to increase or decrease in temperature. As long as the fish are interested, it’s okay. When they’re completely dormant, they won’t come up to feed

  8. emme October 4, 2017 at 9:45 pm

    I see. I’m going to observe that fish a bit longer then. Will let you know if/when I can finally get that limestone, as their hours are identical to my work hours and the commute is very long. Crushed coral maybe I can get in a few weeks.

    The temperature is a lot colder now. It ranges from 8-16 degrees Celsius. Can we stop feeding now? Or still feed once daily? I think Dad just feeds wheatgerm once daily from time to time. Sometimes not at all if too cold in morning.

    The fish do seem much slower in colder temperatures. I’m wondering if the High surface action in the water would be too much for the fish in colder temperatures when they don’t move much??

  9. Venus October 2, 2017 at 9:12 am

    The pink you see in Toby’s fish’s tail fins are caused from nitrite poisoning. The symptoms show long after the fact. Because nitrites rob the blood of oxygen, the blood thickens, and the blood vessels become enlarged, and very visual. The tail fins are the first to show, but if you see tiny patches of veins on the body, this indicates the case was more severe, and the fish may need treatment. The cure is heavily oxygenated water, which you’re providing. It will take several weeks for the symptoms to disappear

    It seems we discussed the idea of crushed coral, which may buffer the water quicker than limestone. Due to the fact your parameters are so low, I might do both. You need gravel on the bottom. This gives waste a place to settle, and helps keep the water clear. Even if you use both, there’s a very good chance you’ll still have to buffer, but, not as often, not as much. Less and less over time

    Some complain that the crushed coral clouds the water, so rinse it off, and leave it in the bag. Place the bag close to the fountain; close to moving water

  10. emme October 2, 2017 at 2:18 am

    Actually, instead of limestone substrate for calcium, can I use crushed coral? I found a decent aquarium supply shop downtown and it’s easier to get to for calcium if coral is a viable source! Not sure how much to use/buy though…

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