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 August 29, 2017 at 8:24 pm

    Interesting. How does green water/algae get cleared over time with pumps??
    Thanks for your help Venus, we will be more prepared with the pumps when winter comes this year.

    The most adventurous food we have fed is blanched spinach and peas. Is lettuce as nutritious? Can lettuce be fed raw without blanching?
    Watermelon can just be cut up and thrown directly in the pond??

  2. Venus August 29, 2017 at 2:37 pm

    It’s not the oxygen that clears the water, but the movement of the water, which inadvertently allows the water to become oxygenated. Surface movement pulls oxygen from the air above into the water, but only if the body of water is clear

    They say moving water doesn’t freeze, and this is true in most cases. If the movement is powerful enough, and the freeze isn’t too deep. With the winters you guys get in Canada, you would have to have a really powerful pump for the water not to freeze

    They say that a hole must be knocked into the ice in order for the water to breathe, but this isn’t true. If your water is properly oxygenated when it freezes over, it will be oxygenated when it thaws, as your fish become dormant in cold water, using very little oxygen. Your pond is deep enough so that the very bottom will not freeze, and your fish can withstand temperatures above freezing

    The cords to the pumps need to be wrapped to minimize exposure to weather. This keeps them from becoming brittle and cracking. Warm or cold weather ages them. Other than that, if the pumps are on the bottom, they’ll continue to operate, because the water won’t freeze below your freeze line, however, the movement is pointless. Unplug them if the surface freezes over completely, but don’t remove them; just leave them in place. As soon as the water thaws, plug them in. They should be on a surge protector if they’re not already

    Fish don’t hibernate, but they do experience dormancy. Their body temperature is the very same as the water they live in

    When the water temps drop to 64f, reduce feeding, and switch to foods that are easily digested; high in roughage, such as peas, gel food, blanched spinach or lettuce, and toasted wheat germ. I’m sure there are many other foods they enjoy as well, such as you’ve mentioned before; watermelon? Stop feeding at 54f as the digestive system is too slow to get the job done

  3. emme August 29, 2017 at 1:15 pm

    Great. I’ll keep monitoring the water parameters. With the addition of the new pumps, the water looks clearer. I think the improved oxygen is clearing up the green water?
    Will keep monitoring in case those extra 10 koi brought anything weird to the pond with them!
    My parents want to add one more pump because the pond guy wants to use it to vacuum up debris to facilitate filtration….? The pond filtering system hasn’t been working very well lately.

    Side note: Last winter was especially cold in Vancouver and the temperature was consistently around negative 10-15 celcius. The pond froze over, the waterfall didn’t work (also frozen) and the fish hibernated under the frozen pond surface as usual. We only had one submersible pond pump last year in addition to the waterfall. So when everything was frozen, the pond pump stopped working under the ice sheet and was broken when the weather warmed up. The fish survived though. We only replaced that pump this summer when my parents returned from the vacation and I started helping out.

    This winter, we will have 3-4 pond pumps submerged. Once the pond freezes over should the pumps be unplugged? At what temperature should pond pumps be unplugged? Or should they never be unplugged regardless of freezing temperatures?

  4. Venus August 28, 2017 at 8:42 am

    I think you can wait for a spell before adding any more remedies. Just keep the water quality up, and things will turn around. The fish are feeling much better already. Just like our hair and fingernails, it’s going to take time for their scales and fins to repair

    The timing is good. The fish will be in great shape for winter

  5. emme August 27, 2017 at 9:57 pm

    Hi Venus,

    All 10 fish were added to the pond. We now have 2 1800gph pond pumps and 1 240gph pond pump submerged in the pond. There’s also the big pond pump behind the waterfall. I added soda and Epsom as usual and fed spinach. Hopefully all will be well. I have 100 tea bags left over. If anything goes wrong or fish get more sick, shall we do another round of mint?

  6. Venus August 27, 2017 at 7:55 pm

    I would get them in the pond as soon as possible. The water they’re in is low in oxygen. They don’t need anything more than what’s already been added to your pond. They’ll be happy to be released in such a large environment with such good water

  7. emme August 27, 2017 at 7:42 pm

    Parents brought home 10 large koi from a friend’s overstocked pond. The fish are gasping in a box with bubble wands. If we add them to the pond, what else should I add? Dad is asking about salt or potassium permanganate?

  8. Venus August 27, 2017 at 6:49 pm

    Nice to hear your parents are excited, and they should be. At this point I would be surprised if you lost another fish,, but we’re not really out of the woods. Some fish may still be on the edge of good health. Let’s give them every advantage by not stressing the ecosystem. Improving good health takes time

    It won’t be easy, but I would advise your parents to wait until next Spring before adding new fish, and then I would stick to one or two new ones, and here’s why. My guess is, they may have been overstocked. By next Spring the fish will be up to spawning, and your mom could be the proud owner of baby Koi; such fun. When the time comes we’ll discuss how to create hiding spots so they live to join the big school

    Right now the cycle is fragile, the water quality is still slightly unstable, and the fish are still recovering. Let’s not do anything to rock the boat. Next year you guys will have the buffering down, and may have found out if you’re dealing with a leak or not

  9. emme August 27, 2017 at 6:24 pm

    E.g. Things to look out for? Or things to add to the water when a new fish is introduced?

  10. emme August 27, 2017 at 6:20 pm

    Hi Venus,

    After the garlic recipe, I added the mint tea last night. This morning we added 1 extra pond pump. Half of the pond closer to the waterfall has lots of surface action. The other side has less. I think with the extra pumps we can have the waterfall off (it auto turns off when water level low) and see where the water level drops to from the leak without compromising oxygen. That said, can 10,000 gallons be sustained with 2 1800gph pumps and 1 2400gph pump? Or is one more needed??

    Side note: parents are getting excited and want to buy more fish since the last huge koi died. Any advice if we are to add 1-2 koi in a week???

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