Fish Health Problems at an All-Time High!

23 minutes read

If you’ve experienced an extraordinary amount of health problems with your koi and goldfish this year you needn’t feel alone. We’ve seen more ulcer disease, infestation of ich and flukes, internal infections and unaccountable deaths this year than ever before. Customers who have had the same healthy fish for years have been experiencing losses this season of 25% or more without having added new fish or doing anything out of the ordinary.

The only factor that is out of the ordinary (in Georgia, anyway) is the winter weather we experienced in May. April was extremely warm and very lovely! Then May came and winter returned. This was highly unusual, unpredictable and caused major problems with the ponds.

Ponds began their spring transition in April. The weather warmed up and pond water temperatures began to rise. Algae blooms were rampant and normal spring water quality problems caused by the absence of budding nitrifying bacteria were in full swing. Parasites and bad bacteria were thriving while fish’s immunity systems were not yet fully developed. This was all part of a normal spring cycle in Georgia — problems we face every spring and the only thing that set it apart from any other spring was that it was in full force during April instead of May. We just shrugged our shoulders, counted our blessing for such wonderful spring weather and started to enjoy our spring with all the normal problems associated with warming waters.

We did not suspect May to be so cold. Temperatures dropped and stayed chilly all month long. Pond water temperatures dropped and stayed in the 50’s and 60’s. That’s warm enough for bad bacteria such as pseudomonas and aeromonas to live and parasites to thrive, but not warm enough for fish immunity systems to kick in or warm-loving nitrifying bacteria, the good ones, to develop. The warmer waters of April gave birth to the bad elements of the pond while the chilly (but not cold) waters of May protected them.

The increase in fish health problems gave us a bit of a shock though, after analyzing this year’s wacky weather’s impact on the pond, I don’t know why we were surprised. Our poor little fish have gone through two springs this year and have been subjected to all the problems associated with warming waters twice! Plus they’ve lived almost a complete month in the absolute worst water temperatures for their health. It’s no wonder why many of us are experiencing major health problems with our koi and goldfish in summer.

Understanding Koi & Goldfish Health

We (here at Pond Doc’s) tend to get pretty sophisticated with our fish health practices and sometimes forget that new pond owners and those who haven’t ever had problems with fish health may not know how to employ the basics of healthy pond husbandry. Those of us who are old hands at ponding need every now and then to be reminded of some facts we may have lost by not practicing.

These factors all play a part in the overall health of the fish in the pond and have direct bearing on how easily fish can handle problems that arise from stresses brought on by this year’s weird weather.

The Environment — Water Quality

Pond water is to fish like our air is to us. If it’s polluted they cannot breathe nor can they remain healthy. The very first factor we consider when diagnosing health problems in koi and goldfish is their environment. The first thing we do is test for ammonia, nitrites and pH.

Ammonia and Nitrites

Ammonia is caused by fish waste and dying plant material. It’s deadly and burns the gills of the fish. Ammonia is converted to nitrites by the good bacterium, Nitrosomonas. Nitrites are also deadly to fish by penetrating the blood stream and causing nitrite poisoning. The good bacterium, Nitrobacter, converts nitrites to nitrates which is basically fertilizer. These good bacteria are present in warm waters but do not thrive well in the in-between water temperatures of spring.

Ammonia and nitrite problems are corrected by increasing the amount of bio-filtration in the pond and reducing the amount of waste in the water. If you experience any readings of ammonia or nitrites when you test your water do as many of the following as possible:

Make a major water change of 75%. Don’t forget to de-chlorinate the water if you use tap water.

Clean out the muck in the bottom of your pond and any dying leaves in the water.

Add a real bio-filter if you don’t have one or increase the amount of filtering materials. A filter that is located outside of the pond is best.

Promote quick seeding of your bio-filter by adding bio-building products such as BioSeed or MicrobLift.

Stop feeding your fish until your water quality problems are over.

If it’s high nitrite readings, salt your pond with 3 lbs. of non-iodized salt per 100 gallons of water. This thwarts the effects of nitrite poisoning.

Test your water each day until problems are over.


pH should be stable. It shouldn’t go from 6.4 in the morning to 8.2 in the evening. It will fluctuate some during the day with the highest readings in the evening and the lowest at daybreak but should stay within the 7.0 to 7.8 range. If it suddenly drops low to 6.2 or 6.0 it can and will kill the fish in the pond. This is called a pH crash. There are products that raise and lower the pH but do not stabilize. If you have high or low pH readings slowly adjust the pH with pH UP or pH DOWN and when it comes into an acceptable range of 7.0 to 7.8 it should be stabilized with a pH Stabilizer.

A quick note about ammonia and pH.

If ammonia is present in the pond and the pH is low the ammonia will be less harmful to the fish than if the pH is high.

New Pond Syndrome

A new pond is much more likely to experience health problems than one that is seasoned. By “seasoned” we mean that the pond is at least one year old and has built up the biological processes necessary to maintain a clean, healthy environment. This year our new pond owners have been especially hit hard with fish health problems. Here’s why:

Over time a pond builds up a slime layer on its liner that is like gold to the eco-system. Some folks unknowingly set the seasoning cycle of the pond back a step by scrubbing off this slick layer of bio-gold while thinking they are doing good for the pond by giving it a thorough cleaning. A thorough cleaning of an ornamental pond should be getting the muck off the bottom and leaving the sides alone. This slime layer contains (among other good things) nitrifying bacteria, Nitrosomonas and Nitrobacter, the ones responsible for converting ammonia to nitrites to nitrates — in other words, the nitrifying cycle. In winter the bacteria goes dormant but remains alive and more readily establishes itself in spring.

A new pond does not have this slime layer. The nitrifying cycle is not in place so even the slightest amount of waste becomes deadly. Even with a super bio-filter on the system, if the bio-filter is not seeded the pond is not ready for a “normal” fish load. Fish load equals waste which equals ammonia and nitrite buildup. If there is not enough nitrifying bacteria in place to convert the waste to nitrates the fish die. Fish should be added to new ponds gradually so nitrifying bacteria can stay one step ahead of the growing fish population. Overcrowding a new pond can mean disaster or, in the very least, increased health problems with the fish — this often manifesting itself by the fish developing ulcers or gill disease.


Parasites often start a chain reaction. They cause port-holes in the skin of the goldfish or koi where opportunistic anaerobic bacteria can sneak in. Some parasites, such as flukes, actually act as a host to these bad bacteria, infecting the fish on contact. The fish develops ulcers from the bacteria and then fungus grows on the dead tissue left behind by the ulcer. Most often as not, the fish dies unless he is treated.

Parasites thrive in cool waters such as what we experienced during the whole month of May. Again, the fish’s immunity systems are low during that time and they are unable to fight off the problems that parasites cause. If one fish has parasites, all of them do. One of the most common symptoms a fish shows when he has parasites is called flashing. It’s when the fish flips quickly and scratches himself on the sides or bottom of the pond. Parasites are brought in by a number of ways including frogs and turtles — even plants that have been in ponds where fish have been — but most often parasites are brought into a pond by a new fish. That’s why we preach the virtues of quarantine.

Different parasites are treated in different ways and it’s helpful to know which parasite is present on the fish so the correct treatment can be administered. Only a few of the many parasites that attack fish are large enough to see without a microscope. They are easily identified by performing a mucus scraping (biopsy) which does not hurt the fish. Once identified the parasite can be eradicated by treating with the correct method of treatment for that particular parasite. Most people do not have a microscope handy. That’s why we offer scrapings at the store for a small $15 charge.

A quick note about parasites.

Parasites quickly jump ship when a fish dies. That’s why we ask you to bring in a LIVE fish for a scraping.

Bacterial Infections

Ulcers, fin rot and mouth rot are a few of the health problems caused by anaerobic bacteria such as aeromonas, and pseudomonas. All natural bodies of water contain a certain amount of these types of bacteria. A healthy, strong koi or goldfish is able to live peaceably with them as long as the amount of anaerobic (bad) bacteria is not elevated and the fish’s immunity system is strong enough to defend him against them.

Bad bacteria thrives in pond environments that are anaerobic, meaning environments with low oxygen levels. Heavy organics (or muck) on the bottom of the pond are virtual breeding tanks for this type of bacterium. They grow in cooler temperatures than nitrifying bacteria so they run rampant during transitional times when water temperatures are in the in-between stage like we experienced in May.

Bacterial infections such as bacterial gill disease are often attributed to overcrowding the pond. Just as an overcrowded room spreads germs among people an overcrowded pond creates and spreads bacteria in fish. I’ve heard many conflicting rule-of-thumbs for the amount of fish load one can have to the amount of gallons the pond is. It’s easy to say 1 inch of fish to 1 gallon of water — or whatever that “rule” is — but there are so many factors to consider when you look at how much fish load a gallon of pond water will support that one easy rule is not sufficient. For example, a pond with poor bio-filtration cannot possibly hold as many fish as a pond with adequate bio-filtration. Inches of fish won’t work because two fish that are 5” each combined do not have the same body weight of a single 10” fish. It’s more accurate to go by pounds but who knows the weight of each fish? When asked I try to make it as simple as possible but leave myself an out. I say 1 goldfish per 50 gallons or 1 koi per 100 gallons (because koi generally grow larger than goldfish) and that “rule” applies IF you have good bio-filtration.

A quick note about how many fish can fit into a pond.

Fish grow. The amount of food they are fed dictates how fat and how quickly but the fact remains that a small fish will grow into a large fish. Plan accordingly not to overstock with smaller fish.

A fish undergoing stressful conditions such as poor water quality, parasitic infestation or overcrowding is subject to fall victim to bacterial infection. The stress factors must be eliminated before bacterial problems can be addressed and cured. Ulcers, fin rot and mouth rot are external infections and can be treated with topical solutions such as dips and water treatments. Internal infections and bad cases of ulcer disease may require antibiotic injections. Either way, it’s best to feed the fish antibiotic food to help them overcome their bacterial problems.

Strong, Weak and New Fish

Some koi and goldfish are stronger and healthier than others just like some people are stronger and healthier than others. The age of a fish has a lot to do with the mortality rate. A certain percentage of babies and small fry will be taken naturally by Mother Nature. The old saying that the stronger survive is especially true with fish. When a koi or goldfish is purchased young, in the first year or first month of life, the chances of losing that fish is far greater than the chances of losing an older fish. Mother Nature has yet to make her natural selections.

Fish become weaker when they are subjected to ammonia or other stress factors. Often this weakens the fish for life. For example, a fish who’s gills have been damaged by an overdosed potassium permanganate treatment will always have scarred gill tissue and it will be a little harder for this fish to survive in adverse water conditions than a fish who has not been through the ordeal.

Fish that are moved lose their immunity to disease for 72 hours. This puts new fish to the pond at risk of becoming sick or dying from stress factors already present in the pond. The “new fish” is branded as being sick and contaminating the others because it died first. That’s not always the right assumption.


Viruses are one of the most dreaded of all fish health problems. Some kill every fish in the pond within hours or days. Some affect only the weakest fish. A virus is not easily detected nor is it easily treated. It is almost always brought in by an infected fish though a wet net can transfer the virus as well. Quarantine with water temperatures of about 70° is the ONLY answer and sometimes the only treatment is to destroy any fish that has been exposed to the virus.

Viruses lay dormant in a fish through the winter and as soon as the water reaches a certain temperature in spring it suddenly becomes active. That’s why we suggest quarantine tanks be heated. To thoroughly rid your pond of a virus you must sterilize the filter as well as the pond.

Fortunately viruses are rarely found in retail outlets like ours who follow strict quarantine processes. They either die in transit or are detected and destroyed at the point of origin. That is not to say, however, that you will never come across a virus. Always quarantine new fish no matter who they come from!


A fish’s body is controlled by water temperature. When water temperatures suddenly heat up something inside the fish is triggered and the reproductive cycle is kick-started. Eggs develop inside the female and spawning occurs. Several males chase the female and literally beat the eggs out of her, pushing her against the side of the pond and into any other object that happens to be in the way. The trick is to push the eggs out of the female so the males can immediately fertilize them.

Spawning is a stressful time, especially for the female. She may jump out of the pond to escape the courting males. Often a female is hurt during spawning by being slammed against jagged rock surfaces. Abrasions and cuts occur often — and often become infected. Unfortunately spawning takes place at the same time when the fish’s immunity is not yet fully established and bad bacteria is growing at an alarming rate. The same abrasions might not be so easily infected if they occurred later in the season when the fish’s immunity is stronger.

Spawning is a very natural and necessary part of life in the pond. If the female develops eggs but there is no spawning taking place she can become egg-impacted, meaning the eggs are not expelled or absorbed into her system and rot inside her. She is sure to die if this happens unless she can get rid of those eggs.

Mysterious Deaths

The hardest fish deaths to understand are the mysterious deaths that leave no signs or clues to what caused it. Several koi and goldfish die for “no apparent reason”. Ammonia, Nitrite and pH tests are normal. There have been no new fish introduced that could carry a virus. A biopsy performed shows no signs of parasites. The gills of the newly dead fish and other live ones look like beets (like they should). Fish health professionals, like The Doc, are reminded just how hard it is to find the answers to every situation.

After conventional testing shows nothing wrong we must explore other less-common avenues.


What doesn’t bother us may kill our fish and often we have no idea the fish is being exposed. Here is a short list of everyday poisons that can affect the fish in your pond:

Copper Sulfate — Children (and adults) may innocently throw pennies into the pond. This can kill the fish or make them gravely ill. A copper fountain can be the culprit of unexplained illness and death. A build-up of heavy metals in the water is a type of slow poisoning. It is a good idea to make periodic water changes — even when you think the pond doesn’t need it. There are also heavy metal removing agents that are commercially available.

Plants — Just as some plants are toxic to us some are toxic to fish and should not be planted near the pond. A few of these are all parts of Foxglove and Lily of the Valley, the berries of English Ivy and the leaves and stems of a potato plant, including a green potato. Midwest Pond & Koi Society has a great listing of these. Find that list at:

Insecticides — Spraying insecticides near a pond, especially on a windy day, is like force-feeding poison to the fish. Minute particles are airborne and can simply settle onto the surface of the water and contaminate the pond. Runoff from a neighboring property can contain harmful insecticides and if your pond is in the path of the runoff it will affect the health of your fish. We have concerns that with increased amounts of insecticides being sprayed for control of mosquitoes we may see more cases of mysterious fish illnesses. That, however, needs to be investigated in detail.

Many times poisons attack the nervous system and cause erratic movements. The fish may swim in a spiraling motion. He may jerk or flip. If you suspect your pond has been poisoned an immediate complete water change must be performed then we hope for the best. Usually, if the fish has been affected by the contaminant the damage has already been done but the change of water will help the others who are not yet showing symptoms.

Electric Shock

Underwater lights or malfunctioning submersible pumps may leak low amounts of electricity into the water. Depending on how severe the leak is, it can cause damage to the fish, deformity and/or death. Electricity attacks the nervous system so the fish may show the same signs as if he’d been poisoned. Often electric shock will manifest itself by the classic crooked back.

The pond can be struck by lightning and this occurs much more often than one would suspect. We had it happen to us. The morning after a storm we noticed two of our koi with crooked backs. A month or so later one had straightened back out and he is still alive and well today. The other was not so lucky and began a quick decline of health and finally died.

Advanced Testing & Diagnosis

Let’s face it. There’s not a lot of money available for research and development for cures for the health problems of ornamental fish. Research dollars go to human diseases and food fish first. Often an advancement in koi and goldfish healthcare stems from a discovery made while searching for treatments for something else. Less than 10 years ago one could find little information on how to treat disease in koi and goldfish. Fish health professionals have made major advances during the last decade but it’s never enough to cover everything.

There are no easy testing methods for poisons in the water. One would need to suspect a certain type of poison then have tests run which are expensive. Testing facilities for poison are less common than facilities that can do simple mucus scrapings and even those are far and in-between.

Viruses and bacteria are cultured. Universities can run cultures but that too can be expensive and inconvenient. It takes weeks to get results which is not practical when the fish are dying within days.

Most times we’re able to find the problem (or a good indication of what it is) but sometimes we can’t. It’s frustrating, especially to people like us who spend a great amount of time and energy with fish health issues. What it boils down to is maintaining a healthy environment in the pond so if there is trouble the fish will be in the best shape possible to handle the stress.

Case Studies

The following are actual cases we’ve encountered recently. The names have been changed.

Spawning First / Infection Later

Sue’s 2500-gallon pond is at least four years old, seasoned and, for the most part, has been trouble-free. It contains many large koi, some that have been there since the pond was first built. This year she brought in a large number of her koi to be placed in our care because they were suddenly all coming down with ulcer disease. Sue couldn’t understand why this year she should have such an outbreak of ulcer disease when she has done nothing different. She hadn’t added any new fish. Her water quality was excellent. The gills were fine. There were no parasites.

Upon examination of the fish The Doc noted large abrasions and cuts on the koi, many of which were becoming infected. The Doc suspected that spawning was causing the abrasions and bacteria was getting into the koi through the skin abrasions. Koi spawn every year. Why should this spawning create such an outbreak?

Sue told us that she thoroughly cleaned the bead filter on her system, carefully following our directions on performing the yearly maintenance necessary to keep the bead filter healthy. Then we got down to brass tacks! Sue was wanting to redo part of her system and explained to me that her pump was only pumping 2000 gph through the filter. 2000 gph is wonderful for filtering through most bead filters but is not strong enough pressure to backwash properly. Bad bacteria thrives in mucky conditions. If the filter is not being backwashed properly muck is accumulating and harboring bacteria. The fact that the immunity systems of the koi were not up to par because of the cold May we experienced along with the fact that bad bacteria was thriving during this time was enough to kick start an outbreak of ulcer disease that took advantage of “battle wounds” inflicted during spawning.

The cure for Sue’s pond was to place a stronger pump on the system and treat the symptoms of ulcer disease. We’ve been treating her koi with our new Tricide-Neo antibiotic dip and they are healing nicely and will be ready to go home soon.


John and Cindy’s pond was several years old, seasoned and we had just conquered the green water problem by adjusting the flow through the UV light. Everything was looking fine and the fish were happy and healthy. They excitedly added an new member to the fish family, a nice GinRin Chargoi picked out while visiting the store.

Within days we received a call that one of the fish (one that had been in the pond for about a month) was not swimming properly and was mostly laying on its side on the bottom of the pond. Tests indicated no problems with ammonia, nitrites or pH. Nothing was out of the ordinary.

They performed a major water change. Two days later two more fish became inflicted with the same symptoms, one of which was the new Chargoi.

The Doc examined the gills of the ailing koi and found nothing wrong. No parasites. The original afflicted koi was still hanging in but was not improving. Steroid injections didn’t help. The erratic behavior of the koi suggested a problem with the nervous system. Without having more sophisticated testing available to us we had to guess - albeit an educated guess. We figured it must have been some sort of poisoning but we weren’t satisfied that poisoning was the true answer. The pond was raised so runoff wasn’t a possibility. How would the poison get into the pond?

Another opinion suggested that electric shock might be the culprit. A short was located in an underwater light supporting the assumption. The light was taken out of the pond and no other koi have shown any of the odd symptoms since.

New Pond, Dying Fish

Brenda had a new pond built that was state-of-the-art and as maintenance-free as one can get. She wanted a great environment for her koi. She was pleased with it’s construction. It was beautiful so Brenda began adding fish. Then it started to cloud up even though it had a working ultraviolet light. The water wasn’t green but it wasn’t clear either. It sounded to me like a bacterial bloom, a common occurrence in new ponds and aquariums. I told her what I would tell anyone with bacterial bloom — to be patient and it will clear up soon.

Then her fish began getting ulcers. Upon testing the water we found the pH to be high so we forced it down and stabilized it. The water still wasn’t clear and was taking on a tea-color. Brenda also noticed that when she backwashed she was getting only clear water out from the waste port. The Doc found the bead filter was not getting the proper amount of flow during backwash and made adjustments. Still Brenda lost fish. Upon examination of the gills we saw signs of bacterial gill disease. Brenda had introduced no other fish into her pond other than the ones from our place and we knew we had no problems with BGD in our stock. This was becoming difficult, even for seasoned fish health professionals like us. Imagine how Brenda was feeling!

The only thing we could come up with was that the fish were weakened by pH fluctuations and worsened by the fact that May was so cold that the fish’s immunity systems were not established like they should have been. This, compounded by New Pond Syndrome, the pond not being seasoned yet, was enough to foster bacterial gill disease. But that is only a hypothesis. We still, to this day, don’t know exactly what happened.

Seasoned Pond, Dying Fish

Rick’s pond and many of his fish were over ten years old and he never had a major fish health problem. He had the same pad & brush filters as always, added no new fish, and did nothing different this year. His fish suddenly started dying and all water tests showed normal. No parasites. No detectible stress factors. Upon examination it was determined that the fish had bacterial gill disease. The koi had outgrown the small pond, the environment had become overstocked and the filter could no longer support the load.

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