Koi pond with floating ice and koi swimming

It’s approaching that most wonderful time of the koi keeping year. Outdoor temperatures are starting to rise and with them your pond’s water temperature. Ice melts, birds come back and flowers start to bloom.

As wonderful of a time as this is for koi enthusiasts, it can also be one of the most nerve-wracking. Rising water temperatures mean more active fish—as well as microscopic organisms.

It is these organisms that can cause havoc with ponds and koi alike.

So how do you go about giving your koi the best shot at avoiding a bacterial onslaught? First, breathe. Then prepare.

Koi are poikilothermic. It means that their body temperature is directly influenced by the temperature around them. As the water cools, so do all the body functions—the immune system included. Once winter is in full swing, a koi’s immune system is all but shut down. And it takes a while to get back up to illness-fighting levels.

An immune system that is not quite up to snuff is susceptible to infection from any host of bacteria, viruses or parasites, but it is Aeromonas that can be the most problematic. This is why it is important to limit any conditions that promote the growth of Aeromonas.


Slide of Aeromonas hydrophilaYes, it is everywhere. All the time. If you have a pond, you have Aeromonas. You don’t get rid of it, you only limit it and prevent it from causing infection.

No matter how good your water parameters are, or how hyper-vigilant you are, there are other vectors that can introduce bacteria into your pristine pond. Birds or even frogs can be the culprit, too.

When it’s not being an opportunistic pathogen, this gram-negative rod bacterium performs a valuable service by feeding on your koi’s waste.

But given the right environment, a single live Aeromonas cell can replicate to 30 million in one day! It is this onslaught of bacteria that causes Ulcer Disease (also known as Hole in the Side Disease).

There can be other microbes and parasitic ne’er-do-wells lurking in your pond as it springs back to life, but Aeromonas is one to watch closely.

Clean the Pond

Seems like a fairly straightforward step. Yet many feel that the big clean before the winter should suffice. Well, it’s better to be safe than sorry. There can be all manner of stuff that can make its way into your pond, as well as the stuff that you might’ve missed cleaning it in the past.

Use pond vacuums, nets or whatever your preferred (and most effective) methods of getting the stuff that shouldn’t be in your koi pond out.

And don’t forget your media either. Your beneficial bacteria will also need a bit of a refresher as they prepare to help keep your water parameters in check. Proper dissolved oxygen levels will help these aerobic microbes. As the water temperatures increase, so will their activity. Again, try to limit the amount of waste in your pond so that the bacteria aren’t overrun.

Water Changes

Your pond has spent the better part of five months accumulating different forms of debris and potential pollutants. Additionally, if you have ice on your pond, the water chemistry will change as it thaws, and will potentially introduce pollutants into your pond that it has picked up.

Changing the water is an important way to remove some of the contaminants, nitrogen byproducts (ammonia and nitrite) and organic waste that might be in the pond.

One word of caution is not to overdo it. Anything more than 20% over the course of a week can have negative outcomes. Removing and adding too much water can have a few negative side effects. For one, it can alter the water temperature, which can be stressful to the inhabitants. It can also introduce high levels of chlorine and chloramine, both of which can prove deadly to a fish that is trying to shake off the effects of winter.


Gas exchange is an important part of ensuring that your koi are safe through winter. However, once the water temperatures start to rise, getting dissolved oxygen levels up is a key factor in limiting the growth of any anaerobic miscreants. Properly oxygenated water (dissolved oxygen) helps to diminish potential health issues.

In colder climates, remember to turn your pumps back on when the water temperature reaches a minimum of 50ºF. If you have water features that help aerate, get them up and running too.


One of the most common mistakes made by enthusiastic koi owners is to start feeding their koi too early in the spring. This is often compounded by overfeeding.

Introducing organic contaminants in the form of uneaten food or higher amounts of waste will allow bacteria to thrive. Add this to the fact that koi will not have a fully-functioning immune system to stave off additional opportunistic pathogens and it’s a recipe for bug disaster.

Types of Food

Next Day Koi's Probiotic Plus Koi and Goldfish foodAnother consideration is what to feed. With antibiotic food use being restricted to vets only in the US by the FDA, boosting the capacity for a koi to fight off any potential illness or parasite becomes important.

This is where preventative as opposed to curative goes a long way. Probiotic food can be instrumental in propping an under-functioning immune system while it gets up to speed.

Blue Ridge Koi’s Probiotic Plus has a blend of Primilac, Vitamin C, and montmorillonite clay—all of which work together to give the koi’s immune system the extra oompf it might need to keep itself healthy.

Dealing with the “New Koi Itch”

It’s a real thing! After a long winter of dreaming about that next gorgeous koi, spring offers koi keepers the opportunity to scratch that itch and add some more color to the pond.

As exciting as getting new arrivals into the pond can be, an ounce of prevention is definitely worth a pound of cure.

No matter how credible you think your source is, you should always put your new koi into quarantine. The recommended period is 30 days. This may seem a little excessive until you discover that your entire pond has to be treated for an illness or outbreak.

Let it be known, Next Day Koi are HUGE advocates for quarantine. Despite the fact that our koi come from a biosecure facility in Israel and one of the most respected koi farms in the United States, we STILL recommend that you put any new fish from any source into quarantine—and that is AFTER we quarantine all our koi for 14 days!

TIP: When you have your koi in a quarantine tank, raising the water temperature to the optimal range where any latent “bug” or disease thrives will speed up the life cycle so it will show up more quickly.

Diagnosis and Treatments

There are a few tried-and-true staples that can help any koi keeper in a pinch if there is a bacterial or parasite outbreak. Potassium Permanganate, salt, and topical antibiotic ointment are a few.

The use of salt as a prophylactic measure will probably be debated until the end of time and is used at the discretion (or experience) of the keeper. As are any other methods of dealing with disease or infection.

It is also good to remember that these remedies should not be used as preventatives. Hold off on adding a lot of chemicals if possible. Your pond will have its own natural cycles and will start to go through them. Adding unnatural additives can be disruptive.

A microscope is also great to have on hand. It helps to take the guesswork out of diagnosing parasites. We outlined the use of a microscope in finding parasites.

If anyone ever told you that koi keeping was a cinch, you need to have that person examined! But the beauty and tranquility that it brings are nearly always worth the uphill. Be prepared and be vigilant. But remember to enjoy your koi while you do!

4 responses

  1. Salt is a very effective treatment for many koi disease and parasites and fungi if you I u find a way to regulate the level of salt which is really a simple process

  2. Nancy Stirling :


    I went through the aeromonas alley scenario combined with (gotta get more) Koi fever last spring and had a harsh learning experience losing a bunch of fish. I treated with salt, potassium permanganate, Proform (malachite green and formalin), treatment for flukes, for anchor worm, for columnaris, used Dimilin, methyene blue, nitrofurazone, prazi, you name it and I spent a fortune on it…..

    But I didn’t (in hind sight) treat the main culprits: water quality and temperature.

    I went through several formalin severe treatments and thought I was OK until this early fall when I lost a couple more fish. I used a medicated fish food from the vet and mixed by my Koi guy here…

    Was unable at the time to raise the water temperature to 85 degrees and hold it there for two weeks. Had a heated quarantine tank I used for the little koi and on the big pond my angel husband built a double layer greenhouse cover with a blower to inflate between the two layers, put a 1000 watt “submersible” heater in there with a temperature thermostat regulator to keep the temperature at a certain level… And we were OK for a while.

    But it started to get colder here in Utah and I was unable to maintain my ideal temperature of 70 degrees at night so after several tries by my koi guy using refurbished hot tub heaters that didn’t work (at all) we finally threw in the towel, said “thanks for your efforts and keep the money” and we bought a 5.5K watt swimming pool inline electric heater (for $150 new on eBay), installed it easily after running the 220 line to the pump house and had 71 degree water (actually 72.6 according to the other thermoregulator that was used as a thermostat in the pond itself) for the rest of this winter and now even into late spring. I fed my fish and interacted with them a couple times every day even on the coldest crappiest blizzard days (and nights thanks to the LED spotlights my husband installed so we could see 5 foot deep easily).

    And my Koi GREW!!

    So… I did keep my salinity at 0.7 to 1.0 most of the winter and did 10-20% water changes weekly depending on how much I fed them and depending on water parameters such as ammonia, etc…

    My little late summer 5 inch tosais are now pretty much doubled in size and the rest of the fish going on their second and third summer in this pond are getting huge. My water quality has been easy to maintain and I’m happy with my pond and the koi that are such a big part of my life.

    Now, don’t do as I do but, I don’t follow normal stocking rules as I have a 2300 gallon pond with an 8000 gallon (purported) bead filter and a 10,000 gallon (purported) home built beautiful bakke type drip filter that I restarted in April. I have about 35 koi now ranging from 24 + inches down to 8 inches. Like I said I don’t follow guidelines well. But knock on wood my fish are all healthy and happy getting fed 2-5 times a day with different foods (Blue Ridge growth food, color food, dried mealworms, dried krill, lettuce, oranges and their favorite food: Cheerios, which I use for hand feeding training).

    I guess in a wandering way- my point is that I believe heat helped me a lot. Yes, it cost a bunch more in electricity. But my bills only went up about an average of $100+ a month over the past 4 months. So about $400 dollars for peace of mind and interactive, healthy feeding fish all winter.

    How much are your fish worth to you? I guess I’m a softie because I am willing to spend what it takes to keep my dog, cat, birds, and fish healthy and keep me happy.

    And I love going out there anytime of day or night and calling “Fishes!!” and having them all swarm to me at the side of the pond and feeding them in the dead of winter when the year before they were hunkered down at the bottom of the pond for the whole winter.

    This is my humble opinion but it sure changed the amount of joy I get from keeping Koi.

    1. Wow! What a story Nancy.

      Yes, you are correct, heat treatment is an often overlooked tool in the Koi keepers arsenal. Unfortunately most hobbyists don’t have the ability to heat, and are not lucky enough to have a partner will to build them a greenhouse from scratch 🙂

      And it sounds like you learned an important lesson (just like you said), that you can throw all the treatments in the world at ulcer problems, but nothing will really get better until you address the underlying water quality and/or stress issues that are allowing the ulcers to happen in the first place.

      Thanks for your post, this is valuable information for others.

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