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 (50,000 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




unnamed IMG_4931 IMG_4935 IMG_4937 IMG_4929 pond pic


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  1. Venus August 9, 2017 at 4:36 pm

    Looks like a good pump alright. Every pond pump must have an aquarium safe sponge wrapped over the intake. Most quality pumps come with cylinder shaped sponges that slide on and off for cleaning ease. The sponges keep the pumps running smooth, as well as clear debris from the water. They should be rinsed in old pond water weekly, or when the action slows

  2. emme August 9, 2017 at 3:53 pm

    Hi Venus,

    I think it’s a combination of leakage and evaporation for the KH. I’m just surprised evaporation isn’t affecting GH.
    Yes, the one pond pump we have is just creating waves at the surface, no fountain.
    We would like to buy more pond pumps because one doesn’t seem to be enough. Since the existing one we’re using is PondMaster, we’re looking at others on Amazon.
    What do you think of this one:
    Or this one:

    Our current pump doesn’t have any controls or sponge

  3. Venus August 9, 2017 at 7:41 am

    Good to know the KH is holding its own. This is really going to change things for the fish. Do the pumps have pipes on them to create fountains? If so, remove them, as these lesson the type of action you really need right now. We need to make waves

    Water evaporates, minerals don’t. They remain in the body of water, so if evaporation is the only issue, you’ll see minerals climb. Maybe you don’t have a leak at all. One would expect to see all minerals climb in value over time with evaporation. I can’t say why GH is dropping and KH remains the same. Your mineral values were so low when you came on board, maybe this in itself tells us you have a leak. When did the evaporation start? It could be a combination; evaporation and a leak

    Rinse sponges as the action slows. Did you see volume controls on the pumps? Are the sponges on the outside of the pumps; over the intakes? This is where they should be, not inside the pumps

    Let’s see a pic of the pond’s surface movement after lowering the pumps and removing the extensions. You can email it to me, and I’ll attach

    No, it’s no fun having to drain a pond of this size, but it can be done if need be. We’ll just keep tabs on it, and if when cool weather sets in, the water table is still lowering, we’ll know it’s not due to evaporation

  4. emme August 9, 2017 at 12:21 am

    I’m wondering also, why KH is staying relatively buffered with the pond leak and evaporation, while GH is decreasing? Or maybe the question should be why does KH increase with evaporation?

  5. emme August 9, 2017 at 12:16 am

    Hi Venus,

    We lowered the pond pump to the deeper middle part of the pond and hope to buy another pond pump soon. The surface action at the top of the water definitely reduced due to the depth of the pump.

    KH has maintained at 100 without any additional soda but GH has dropped quite a bit so I added Epsom to target 180 again.

    We have somewhere between 50-70 fish I would say in this pond. They are all different sizes though. Some are huge (the length of an adult forearm) and the smallest are small enough to fit in the mouthes of the huge koi. The idea of draining a pond and reestablishing the cycle like your friend’s pond sounds very stressful.

    I think at the moment I’ll try to improve the water quality and wait for more pumps to arrive and maintain the KH/GH with those pumps before adding treatment.
    Since learning about the pump situation I didn’t bother with the mint tea.
    But once the new pumps are set up, do you recommend I use mint or your new garlic recipe first when it arrives?

  6. Venus August 6, 2017 at 7:25 am

    Anaerobic bacteria simply put, are bad bacteria

  7. Venus August 5, 2017 at 10:48 pm

    The pumps must sit on the floor of the pond in order to remove gases from the water. Co2 hangs heavily on the bottom where waste settles. I would bet the pumps are pulling a lot of it up, but considering you’re feeding on the quiet end; the fish are probably spending a great deal of time in water that’s low in oxygen

    Next time you test, pull water from the depth instead of the top. In most cases you can get a good general reading of the pond water from water at the top, but obviously, not in this case

    Tonight I visited a pond very close to your size if not a little bigger. They had a major kill, drained the pond, and in the process the pumps dried. The cycle broke, and during the cycling event they lost a lot more fish

    They’re really lucky. The town they live in has excellent water quality; top of the comfort zone for GH and KH. Their only problem is not enough water movement. They have green water algae, and a little blanket algae as well; easy fix by adding additional pumps. These types of algae lower oxygen levels by taking up space, much like Co2

    They’re having a lot of evaporation right now, and KH is really high, but the fish are taking it well

    They were grossly overstocked with over three hundred Koi. The fish were only a foot long, but you have to consider the fact they’ll grow up to three foot long, and have offspring. I think they’re down to about 75 fish now; just about right for the pond size

    Water movement is all important for so many reasons

  8. emme August 5, 2017 at 6:03 pm

    If not algae, then anaerobic bacteria? Would it help to first install proper pumps on the pond floor? One at the shallow end and another on one of the sides of the deeper middle pond?

    • emme August 5, 2017 at 10:23 pm

      I tested the pond parameters on 4 corners of the pond and they appear to be consistent. I do see the fish staying at the top of the water appearing to gasp for air though. This is when I’m not nearby and they can’t see me. Some koi hang around the water pump near the shallow end and are pretty motionless (float in one spot) until we walk to the pond and then they begin to swim and beg for food

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