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. Venus August 25, 2017 at 7:25 am

    Everything you’re doing will work to kill fungus; raising KH and pH, as well as the remedies. Bad bugs can’t tolerate heavily oxygenated water. Oxygen destroys them. Yes, I would add the mint. Mint is a good follow up for garlic. Switching remedies is important, as bad bugs quickly build immunity. The only thing they can’t build immunity to is oxygen

    Yes, add the pond pumps as soon as you can. The extra action will do the fish good. Leave a 10’x 10′ space at the quiet end for feeding though

    Once you get those pumps set up, watch for the action to slow; meaning the sponges are getting clogged. Very important to keep them clear, and with all that green water algae, you’ll need to rinse often

    The healing process is slow but sure. You’re fish should be fit as a fiddle by the time cold weather sets in

  2. emme August 25, 2017 at 1:18 am

    Hi Venus,

    I finally got KH back to 110 and GH to 180. I added a small amount of soda and Epsom to buffer, before adding 6L of hydrogen peroxide (mixed with pond water and poured around the whole pond). I then added the new garlic treatment to the quieter end of the pond. Will feed the garlic spinach and gel for the coming days.

    I may add mint tea in a few days. I’m wondering if the garlic and mint will help with fungus though? Some fish have white grey/patches.

    The pond pumps have arrived. Shall we add them next week after the fish finish eating the gel food and garlic spinach?

  3. Venus August 17, 2017 at 7:44 am

    Types of common algae are green water, blanket or string and last but not least, substrate algae. The latter is the only good form. Instead of free floating, it attaches to something. Green water algae takes up space in the water, keeping oxygen from entering. Blanket or string algae covers the surface, keeping oxygen from entering. These types of algae are found in quiet areas of the pond, and most large ponds are going to have these areas

    Most people don’t realize that algae is algae. The only thing that makes the difference is water movement

    Hopefully the new pumps will provide enough action to discourage green water algae. Algae is very good thing, feeding on nitrates, keeping levels at bay, and indicates a strong cycle. This explains your low nitrate levels. All too often pond keepers use chemicals to eliminate the harmful forms, which of course are hard on the fish and the environment; when all they had to do is increase the action. Increasing the action does much more than eliminate these forms of algae as you know; eliminating Co2 as well

    Once the new pumps are installed, watch for the action to slow. This indicates the sponges are filled with algae, and need rinsing

    Your parents are a little too aggressive with cleaning, but with some a little guidance, maybe you can change this. Exchanging large amounts of water can upset the cycle, and in your case, decrease mineral value, and it’s only a temporary fix, as are chemicals. Increased action is the key

    As for the canister, there should be an intake with a screen over it, and this is where the sponge goes. There’s no need to fasten due to the fact the suctioning should hold it in place. It’s a shame really. Most ponds with water falls are set up as a permanent fixture, and it’s a real feat to take them apart. The good news is; most of the beneficial bacteria will reside in the pond pumps, so I doubt cleaning the canister would cause much of a spike. These types of filtration systems attract mostly bad bugs unfortunately

    Water falls can also be created using pond pumps. The arrangement of the stone should be as such, so that hoses are easily pulled and cleaned or replaced. Few pond builders have concern about maintenance. I often wonder if any of these people even have ponds. lol

  4. emme August 16, 2017 at 10:49 pm

    Hi Venus,
    Due to the setup of the waterfall pump and canister, I don’t think it’s possible to attach extra sponges. I haven’t started the treatment yet. I was maintaining KH and GH but my parents cleaned the canister sponges and did some extensive backwashes so I lost what I had buffered to KH below 80 and GH below 140. I’ve been chasing KH and GH since as it has been dropping daily. KH is currently 60-70 and GH is currently 140. I added 750gm soda tonight so hopefully it will make a difference when I retest tomorrow. My parents Donna leashes to clear the water as there is a lot of green algae/water. How can that be prevented? Would the arrival of the new pond pumps help?

  5. Venus August 16, 2017 at 7:09 am

    Back-washing a filter is a term for clearing the debris. Fasten a sponge over the intake hose, and rinse in old pond water, just as you do the sponges on the pond pumps. You should never have to clean out a pump if the sponges are in place. Every time you clean a pump or filter, you risk losing a portion of your cycle, even though friendly bugs prefer pond pumps over canisters

    If your fish were healthy, I would say a KH reading of 80 isn’t half bad, but your fish need better than that. They need excellent. I think you’re a little timid; afraid of going over

    Did you add the remedy?

  6. emme August 15, 2017 at 10:35 pm

    Hi Venus,

    For some reason KH is steadily decreasing with pH in the pond. I seem to have lost the KH buffering point so when I add tiny amounts of soda it’s no longer doing the trick. This has been since KH dropped below 80. I’m working to increase it again

    • emme August 15, 2017 at 10:43 pm

      Could have been triggered by my parents “backwashing” the filter. Quite a bit of pond water was lost and readded. How often should pond filters be backwashed?

  7. Venus August 14, 2017 at 8:10 pm

    That’s just one reason I was thinking of placing them at the waterfall; perhaps in a strainer? For maximum benefit they should be close to fast moving water no matter where you place them. Shells are simply a mineral block that breaks down gradually

  8. emme August 14, 2017 at 5:29 pm

    Hi Venus, I had no idea I had to fish the shells out of the pond once the water parameters were fine. I assumed I could just leave them in there and they would self-regulate…
    I guess not..?!

    • emme August 14, 2017 at 7:07 pm

      Because that is a lot of shells to remove from the bottom of the pond…

  9. Venus August 14, 2017 at 5:06 pm

    I thought you said you ordered the giant size for ponds, so I thought…gee; they’re really small for giant size. lol

    You can add as many as you want as long as you test regularly. They don’t ‘all of a sudden’ break down, or dissolve over night, so pull them when the KH is right where you want it. I can’t say how long it will take; could be much slower than we hoped for. I would guess the Lower the mineral value in your pond the faster the process

  10. emme August 14, 2017 at 4:11 pm

    I just noticed on the wonder shell package that it’s 1 super size shell per 10-20 gallons. The website I ordered from only had super as its largest shell size…
    Venus, do you think 4 shells will be enough? Or should I be emptying a dozen in there? LOL
    I Probably should be ordering the giant size off Amazon:

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