My Koi Just Died and I’m Worried About The Others?

Here’s the deal:
We have four koi in a 55gal tank. One of the koi started losing his colour and stopped eating. For about a week he was at the top looking like he needed air, so we started airating the tank more, but his condition didn’t improve. When he did eat he would spit out every other piece and started convulsing. About a week later all of the koi started gasping at the top. So I changed the water and left out the chemical treatment and all of the koi stop the gasping. The sick koi started getting better for another week, but after that he just stayed at the bottom of the tank and only swam to get food. He was gaining weight and looking better, but then I walked down one morning and he was dead. Should I be worried about the other 3 koi catching what he did?
Okay, I know koi are pond fish, but when you live in an area where four months out of the year water is thawed you kind of can’t. The koi are 5inches and under. We do a water change (25-30%) every week, along with cleaning out the filter, and change the medias every month. We stopped adding the chemicals or salt because it seemed to be making the koi worse.
Haha, last detail:
The water we use to change the tank out with is run through an RO filter, so it is purified water we use.

5 Responses to “My Koi Just Died and I’m Worried About The Others?”

  1. tikitiki says:

    Koi aren’t aquarium fish, they belong in large ponds. They grow to 3+ feet, that’s why they’re not good for aquariums.

    I would say from what you’ve described, it sounds like the water quality was bad-ammonia and/or nitrites, especially since the water change helped for a bit.

    Buy a test kit, and monitor the water quality. Start doing frequent water changes, and consider digging a large pond or relocating the fish to someone who has a pond.

  2. jessica_birdie says:

    How big are your koi? If they are pretty big, I would try to get a bigger pond, 55 gallons is not very big.

    If your other koi are doing fine now, I would think they should be fine.

    I have had sick koi and once he was out of the tank, all the others were fine.

  3. magicman116 says:

    Yes, you should be concerned about they other Koi. Depending on the size of the fish it could be a water quality issue and you may need to start finding them new homes or trading them in at the pet store. Koi will rather quickly out grow a 55 gallon tank and death of fish will be the outcome of the over crowding.

    If they are still small (under 5″ or so) then it’s possible you are seeing the results of a disease instead and in that case you need to watch the others very carefully for any odd behavior. Make note of it and act quickly if you see problems. What to do will depend entirely on what you see and what the diagnosis is for your fish. The information you gave could point to any one of several problems.

    Personally, considering the time frame you mention, I would suggest it was and is, most likely water quality that is the problem. You shoudl be changing at least 25% of the water int he tank once a week and cleaning the gravel well with a gravel siphon to keep the conditions stable in the tank.

    If you have more details or if I can help further, feel free to email me.


  4. Demon L says:

    Koi get to be really big. Most koi hobbyists recommend a 2,000 gallon pond, at minimum. It sounds like you need to do frequent partial (twenty to thirty percent) water changes. I do them every other day, but most people suggest doing them at least once a week. You’ll need to get a bigger tank or a pond set-up. Last, after you do a partial water change, add eleven tablespoons of non-iodized (plain or Kosher) salt to the water. Be careful not to pour any directly onto the Koi. Salt helps improve gill function, and it also kills diseases. Let the salt sit for at least twenty-four hours before doing another partial water change.

  5. Poopy says:

    It is very common for koi to change color over time. Red color, especially, tends to fade over time, and black is one of the last to come in. I have had two-step kohakus turn into three-step kohakus and kohakus turn into showas over a summer, just by the migration/late development of their color. You could try using a full-spectrum (UV) bulb to ensure that, being indoors, they are getting enough UV to develop pigmentation, but this is a fish-by-fish issue not likely to spread among your total population (unless you have very hard water – hard water tends to encourage the development of black spots).

    As for the gasping, loss of appetite, and convulsing, however, I would probably blame one of two things. The first, and most likely, candidate, would be nitrogen build-up in the water – especially since he seemed to get better after you did a water change. So, test your water first. Ammonia and nitrites should be zero, nitrates should be as low as possible but certainly less than 20 ppm. This issue would certainly affect all your fish.

    If your water parameters check out as they should, I would turn to parasites next. Flukes, especially, come to mind, as koi tend to be rather susceptible to them, and if one had them, it’s a sure bet the rest have at least a few.

    Even if it is isn’t flukes, you can do a “shot gun” remedy that will get rid of most everything, flukes included: First, add salt to 0.3 or 0.6% – .3% if you have live plants, or .6% if not. Do not use table salt or anything else containing iodine. Next, gradually warm the tank no faster than one degree per hour, up to the mid seventies, or even up to 86 degrees. Start feeding a medicated food like MediKoi – it is, in my estimation, by far the best medicated food. It will help control any secondary bacterial infections that can develop when a fish is weakend by stress or parasites (if you start to see redness develop in their fins, however, that is a sign of sepsis/septicemia and Medikoi will not control that on it’s own – you have to treat with a good anti-bacterial like AP Ponds).

    Finally, you can combine two parasite medications and apply them at the same time: Dimilin or ProForm C, and Praziquantel. All are widely available over the internet. Dimilin and the ProForm both easily control comparatively rare but large parasites like anchor worms or fish lice, and Prazi has no equal when it comes to treating flukes, which neither of the other two really touch. These are all relatively gentle to the fish and will cure about 90%+ of the most common problems. Prazi is expensive but well worth it, in my opinion.

    Be sure to follow all dosing instructions on the containers, including quantity, frequency, water temperatures, and whether or not you can use them in salted water. For this reason, I usually salt last, as some meds, when mixed with salt, will actually either a) decrease the effectiveness of the meds, or b) turn the meds caustic to the fish’s gills.

    Finally, check out It is run by a veterinarian who specializes in koi, and is hands down the best web site I’ve seen regarding the care and keeping of koi.

    Hope this helps – good luck!

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