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 October 2, 2017 at 2:05 am

    Pond Update! It’s been a week since the pond lining was patched and there has been no leak so far! KH and GH are holding steady so I’m able to test parameters 1-2x/week. After a week of turning the filter back on, the water is much clearer. Pond chemistry has improved and it looks like the good bugs are returning. Ammonia = 0.1, Nitrite = 0.3, Nitrate = 20-50. KH= 120, GH= 260. I think I’m going to target KH to 140 and GH to 300. Why not aim for the best since the leak is fixed right? 😀 I’m thinking I should add calcium instead of magnesium since we’ve been doing all Epsom for GH. I should probably wait for an opportune time to buy limestone substrate??
    The fish indeed seem to be slowly mending. Although, we’ve noticed that the tails of 2 of Toby’s fishes turned pink and a bit frayed. Is this bacterial? Stress from the lowered water table and no filter while looking for the leak before?

  2. Venus September 26, 2017 at 7:26 am

    That’s good news. Things are going to get a lot easier now

    It’s possible the lowered water table increased nitrates. If this were the case the fish would be on the bottom; not swimming around. They could be stressed by all the activity, but if this were the case the stress would be short lived. Lower temps are more than likely to blame. It’s only natural they slow down as the water temps lower due to cold weather

    Keep an eye on ammonia and nitrites in case of a spike. Feed foods high in roughage when the temps are low if they’re interested in eating

    Do you have a thermometer for the pond?

  3. emme September 25, 2017 at 10:49 pm

    Hi Venus,

    After 2 weeks of pond draining, we finally found and patched a hole in the pond lining. The pond has been slowly filled to the top again and the pond pumps are running as they were before. No water leakage has been sighted yet! Currently working on buffering KH back to 120 and GH to 300. I’ve noticed that he koi are a bit slower than they used to be. Is it due to adjustment? Or colder temperatures? It’s been colder recently but weather temperatures will vary from 13-23 over the next week

  4. Venus September 21, 2017 at 7:19 pm

    Yes, stopping the leaks helps all the way around. It’s not easy finding a leak. You may have to fill and then lower it a few times. Just so you know and don’t point an accusing finger at the pond guy. lol I’ve been there

  5. emme September 21, 2017 at 7:08 pm

    Hi Venus,

    It’s the temperature of the air. I’ll add garlic tonight and save mint for next time when it’s warmer. I think the pond not leaking will help the garlic!

  6. Venus September 21, 2017 at 7:13 am

    In a pond this size, it takes several days for the water temps to drop when the weather changes. Is this the temperature of the water or the air? Goldfish and Koi both begin dormancy at 17.7 c or 64f. The lower the temps, the less oxygen they use, the less active they become, so of course treating them with remedies is pointless if the fish are in full dormancy; 54f and under

    When it comes to treating parasites and bad bacteria or fungus, it’s best to treat in warm weather due to the fact the bugs are more active, and more vulnerable. I would say at this point bad bacteria are no longer a problem. Are they present? Sure. Every body of water has bad bacteria just waiting for a chance to thrive, but as long as the water quality is good, they won’t get that chance

    Go ahead and use the remedy now. The temps are still warm enough for them to benefit. Garlic does much more than destroy bad bugs. It has a marvelous healing quality and boost the immune system, something your fish really need right now. Garlic is also good for the heart, prevents disease and kills cancer cells. Taken internally it’s a probiotic as well as an antibiotic

  7. emme September 21, 2017 at 2:44 am

    We’re still in the process of finding the pond leak but the water level seems to have stabilized. The whole pond has been running on 4 submerged water pumps (waterfall off). The outdoor temperature is quite a bit cooler ~13-15 Celsius in the evening when I’m home. Will the h2O2/garlic/mint treatment still work?

  8. Venus September 17, 2017 at 7:40 am

    It’s hard to resist the urge to get new fish. In the world of aquarium fish keeping we call it, MTS; multiple tank syndrome. It happens when the main tank becomes overstocked, yet the fish keeper just can’t stop getting new fish. The same is true of pond keepers, only adding a second pond is a little more complicated

    It’s always good to keep well below the stocking level to make room for small fry. If there are any good hiding places, there will be baby fish, which is more fun than getting a new fish really. If you’re able to add hyacinth next year, the root system makes a great nursery

    How lucky will that be if the leak isn’t at the bottom? I can’t imagine catching up all those Koi and moving them to a temporary place; what a feat

    It’s probably too soon to expect frayed fins to be completely repaired, but you should see improvement. By the time winter sets in, the fish should be in good condition

    Yes, the higher nitrates are due to the lowered water table; higher concentrations of waste compared to the body of water. Continue feeding as you are so as not to disturb the friendly bug colonies. The higher levels of nitrates are going to encourage more algae to form, and then when the table is raised, and the nitrates are lowered, you’ll probably lose some algae due to the lower toxin levels, but this shouldn’t effect the cycle

    When you find and fix the leak (that in itself is no easy task) and are ready to fill the pond, add a little every day so as to avoid a spike. Take a week or two to fill the pond. This too; if the leak isn’t fixed, there won’t be too much water loss to find it out

  9. emme September 17, 2017 at 1:51 am

    I should also add that the higher Nitrate concentration is likely also due to the reduced water level and lack of water topup to dilute… A bit bothersome…..

    • emme September 17, 2017 at 1:55 am

      High nitrate levels may become an issue once the leak gets mended eh? We’ll no longer be exchanging fresh water for leaked Nitrate pond water… hmm

  10. emme September 17, 2017 at 1:41 am

    Haha I’m just pleased the fish are slowly improving. I wasn’t sure we could get this far but I’m glad we persisted.

    The leakage appears to have slowed/paused but we’ll give it several more days to be sure. The water level actually isn’t that low at the moment, but we’ve never let the pond leak this much due to lack of oxygen/waterfall in the past.

    The garlic has not arrived, but I’ll start it once it arrives and use mint after. It will probably be more effective with the lower water level?

    I haven’t been able to examine the fish recently, so I’m not sure how their fins are. It’s dark by the time I get home so I just test the water parameters. KH is still 110, GH 220 (consistenr levels due to no water topping).

    I haven’t followed up with Toby since I referred him to the blog, but I should probably see how he’s doing!

    Nitrates are currently ~50ish with 0-minimal nitrites. Sometimes they get closer to 70 when we feed more. I read online that nitrates below 80 are still acceptable. Is that true? If so, I’m thinking maybe our current stocking levels are at equilibrium with nitrates and any additional push may cause them to get closer to/exceed 80. I’ve actually been telling my dad not to add any fish unless he trades some out/gives some away. He appears to agree….for now.

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