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/Aquatic Gardens, Design, Construction & Maintenance

Koi, Pond Fish Disease

By Bob Fenner

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner

Disease is "any condition that deviates from a normal or healthy state". It might well be considered as a situation out of whack, out of balance. There are several types of and ways to describe disease: social, environmental, nutritional, genetic, infectious, parasitic... This article attempts to delineate some of the more common, easily diagnosed complaints of pond fish keeping, and offers some suggestions on prevention and treatment.

As with humans, fish diseases are rarely caused by single events, nor are their treatments effected by one simple change or addition. Very often water quality and nutrition play a central role in "re-centering" a system, including the living and non-living portions. When your fish(es) seem out of sort, first look to the causes; typically water quality degradation, and rectify those shortcomings, before or rather than resorting to therapeutics.


Prevention is always the best medicine. In an aquatic system, there are three general areas of disease prevention: environmental stress, fungus and bacteria, and parasites.

To overcome new-comer stress use a good conditioner like Novaqua when introducing new livestock; this will neutralize chlorine, detoxify some metals, and provide an extra slime coating for the fish. If a chlorine neutralizer is not wanted, Polyaqua may be used. To prevent fungus and bacteria infections, add about a tablespoon of salt per five gallons to the system. If live plants will be in the system, use only a tenth dose. To prevent fungus and bacteria in a system, use adequate filtration and frequent water changes. Ultraviolet sterilizers, ozone generators, and protein skimmers are high-technology add-ons that can boost water quality and extend safety margins from over-feeding and crowding.

For parasite control use quarantine, prophylactic dips and, if necessary copper. When using copper as a prevention use only a minimum dose, (0.15 ppm copper initial dose maximum)

Do not use copper with invertebrates. Use Tetra medicated food or make your own medicated food when introducing new fish or when invertebrates are present.

If fish still become sick, check for possible causes of stress and eliminate it if at possible. Identify the disease and treat accordingly. Carefully record what you observed and tested for, as well as what you treated with and how.


Any system's condition that is not good for the livestock may cause stress and too much stress usually leads to disease. The most common sources of stress are:

1) Improper pH or drastic and\or sudden changes in pH.

2) Improper temperature or sudden changes in temperature.

3) Improper salinity or water density for extended periods.

4) Improper hardness, or sudden change in hardness.

5) Pounding on the system, or sudden movements that scare the fish.

6) Aggressive pond-mates.

7) Poor diet.

8) High metabolite levels. (A high nitrite level prevents oxygen from reaching the cells and may cause suffocation or brain damage).

9) High nitrate levels. (This may lower the PH as well).

10) Any measurable ammonia level. (80% of all fish waste is in the form of ammonia and is extremely toxic if not converted immediately into nitrite).

11) Other toxins. (Chlorine, copper, detergents, iron, lead, zinc, commercial ammonia, nicotine, perfume or cologne, oil, paint fumes, insecticides including contamination from dog and cat flea collars, etc.).

12) Too little or too much dissolved gas in the water, or a rapid change from water that is saturated with gases to a "normal" gas saturation level. This can be a problem when releasing newly purchased fish into the system. If the water is not mixed, the fish may suffer a condition that is very similar to "the bends" and may be just as deadly. 

13) Too much or too little light. Too much light (no, or short periods of darkness), speeds up the metabolism of the fishes and does not allow the fish to rest. Too little light and the fish may be lethargic and not eat properly.

14) Dirty or cloudy water. Cloudy water is usually caused by bacteria. The bacteria in the water uses all the available oxygen and the fish suffocate.

15) No hiding places for the fish to feel safe.

16) Loss of mucous of the fish's slime coating. This condition may be caused by stress instead of the other way around, but once the slime coating is lost, additional stress is incurred. 

17) Infectious (bacterial & fungal) and protozoan diseases.

18) Any other sudden changes in the environment.

19) Overcrowding.

Stress triggers the release of epinephrine, a hormone which prompts the fish to get ready to fight or flee. (The hormone used to be called adrenaline.) This increases the heart rate, blood pressure and respiration. At worst continual stress will cause a fish to die of exhaustion. At best the fish's immune system may become so over-stimulated that it functions improperly and the fish dies from a disease that would not kill an unstressed organism. A over-stressed fish usually becomes sick. If the stress is severe the fish may go into shock and die immediately.

Most aquatic systems have a constant supply of fungus, bacteria, and parasites that have little or no effect on a healthy fish. Once a fish is under stress however, it may fall prey to the disease organisms that comes along or are already present but that it might have resisted without being over-stressed.

Fish and invertebrates are more dependent on their environment than any of the higher animals. They are totally dependent on their owners to provide them with proper living conditions.



White spot disease. Think of ich (Ichthyophthirius) as an army of individual animals living off the juices of the fish. While on the fish they are usually encysted and protected, and cannot be killed until they drop off the fish to reproduce on the tank bottom. Therefore treatment must continue through the entire life cycle of the parasite which usually is about 4 to 5 days. A low temperature can slow the cycle down to 6 or 7 days. A high temperature can speed it up to as little as 3 days. A high temperature (85 degrees or more) can kill parasites by itself but may also kill the fish or add to the stress of a fish with parasite choked gills.

Any parasitic infection is usually easy to cure if treated quickly with an effective dose of copper. If the dosage is too low, not all parasites are killed and re-infection results. If treatment is delayed, the parasites may become so numerous that they choke the gills and the fish suffocates or the fish becomes so weak it cannot recover. Treatment should continue for at least 4 days and a good rule of thumb is to treat the system every day until no sign of infection is visible, then treat one more day.

Each parasite leaves a wound where it was attached to the fish. These wounds are easily infected by bacteria and an antibiotic should be administered following the copper treatment. Copper levels may be changed by the pH of the system, staying in solution longer at a high pH. It is possible to maintain a safe copper level at a low pH and increase the copper to toxic levels by doing a water change that significantly raises the pH of the water and re-releases absorbed copper back into solution. Chelated copper will remain in solution longer than ordinary copper sulfate. Always use high quality copper to avoid contamination from other minerals that may prove to be toxic.

Tetra medicated food, flake "D", is highly recommended in conjunction with copper, and in a system with invertebrates may be the only available treatment. (Copper kills invertebrates).

Signs of a parasite infestation are:

1) Visible spots, usually white, that make the fish look like has been salted or covered with powdered sugar.

2) Rapid or heavy breathing. Some parasites will attack the gills before any can be seen on the fins or body, and the fish may die from suffocation. 

3) Scratching. If a fish constantly rubs against objects in the system and looks like he is trying to dislodge something, he is probably trying to rub something off and it is probably parasitic.

Because ICH reproduces in the system, the whole system must be treated, not just the infected fish. Hospital tanks and dips seldom effect a permanent cure, and cause a great deal of extra stress for the fish. Ich infections may produce an immunity to later attacks.

Malachite Green is effective against parasites but may damage gills and burn the fish. Malachite green that is not pure is especially dangerous.

Dylox (DTHP) is effective; it is an insecticide and is generally not toxic when used as directed.


This is actually a form of algae parasitic on fishes. Treatment is described under ICH.


These are easily visible and look like little sticks about 1/4" long protruding from the body or fins. They are firmly attached and when pulled out may hold onto a piece of flesh. Medicated food, flake "D" and Dylox is the recommended treatment and it usually takes 10 to 14 days for full eradication. After a few days of treatment, any remaining worms should be removed from the fish. Because of the large sores left by the parasite, fresh water and an antibiotic is a must during and following treatment with Dylox and flake "D".


Are crustacean parasites with similar treatment as per anchor worms. They are about one quarter inch flattened discs with rasping mouth parts and hook-armored legs capable of damaging fins and skin. Treat for secondary infections as well as eradicating these pests.


This is a virus that lives off of impurities in the water while attached to a fish. It does not live off the fish (like ICH), but may kill indirectly by interfering with gill movement, swimming ability, or eating. Lymphocystis can only be killed in an established system by removing its food source by means of purifying the water. This can be done with ultra-violet sterilizers, ozone, chlorine, frequent and large water changes, micron filtration, or diatom filtration. In the late stages of successful treatment, the virus clumps may be easily removed from the fish by scraping with your fingers, or may drop off as it dies. 

The virus itself cannot be trapped by filtration, only its food source can be removed. Lymphocystis virus transfers easily between species but less easily between genera. The more crowded an system, the easier the virus may transfer from fish to fish.


Bacteria grow erratically and are often white or milky in appearance. A bacteria infection may be localized or may be evident on several areas of the fish. Bacteria infections are likely to be found in or around open sores or any area where the fish has lost it's protective slime coating.

Antibiotics and medicated food should be used to treat bacteria infections along with frequent water changes. A dirty system can prevent successful treatment. Because there are so many different types of bacteria, you may have to try several types of antibiotics before finding one that works. Be sure to do large water changes between treatments of different medications. Different resins and high-quality carbons will remove medications from the water and should be removed during any treatment with antibiotics.

Internal bacterial infections may be identified by the gas they produce. This may cause the following symptoms- swelling, a fish that has trouble staying on the bottom, whitish feces that float or trail off behind the fish, or lack of feces entirely (blockage).


Fungus spreads evenly, starting from a central point and growing in an outward pattern. Several areas may grow outward until they overlap and give the appearance of a bacteria infection. Fungus is white with a velvety or even hairy appearance. It is most likely to be found on the mouth, eyes, or tips of the fins.

Treatment consists of water changes, medicated food, and antibiotics. Sulfa drugs administered through food may be your best treatment.


This is a symptom, not a disease caused by a specific organism. It is manifested by swelling behind the eye(s), or in the eye(s). The swelling may be caused by many factors but is most commonly caused by bacteria. If unilateral (one-sided), the cause is probably mechanical injury. Only time passing may effect a cure. It is difficult to treat, but the most effective procedure seems to be a good environment, and medicated food. Start with flakes "D" and "A" in conjunction, then feed flake "B", and go on to flake "C" if needed. Erythromycin or Chloramphenicol may also be effective.


The swim bladder is the organ which allows a fish to stay at any level in the water column without sinking or floating. The swim bladder may fail from damage by bacteria, parasites, genetic faults, or blows and/or bruises. When the swim bladder fails to function the fish loses it's ability to swim normally and may swim sideways or even upside down. Once damaged, the bladder does not usually return to normal function, but if the fish can eat and swim without too much strain it can live for years with the condition. Goldfish seem particularly prone to swim bladder problems. Without knowing the exact cause of the malfunction, treatment is difficult. Since internal bacteria, fungus, or parasites are the only treatable causes, medicated food and/or antibiotics should be tried along with frequent water changes.


Dropsy is a name given to any disease that causes a fish to swell so much that the scales no longer lay flat against the body of the fish. By looking down on a fish you can easily spot a case of dropsy. This is a very difficult condition to treat successfully. Daily doses of Erythromycin, daily water changes, and exclusive feeding of medicated food (D&A) has proven to be the most effective treatment. Including regular feedings of daphnia may prevent some cases of dropsy. Goldfish have the most trouble with dropsy but it may be found in any fish.


Rapid breathing or gulping near the top of the tank may mean a fish is not getting enough oxygen. This may be caused by:

1) No air circulation. Air pump or stones may be faulty or missing. Surface agitation may be missing in a "system".

2) Temperature is too high. The warmer the water, the less oxygen it can absorb and hold.

3) The water surface is covered. Water cannot absorb oxygen if the surface is covered with scum or oil.

4) Parasites. Gills clogged with parasites cannot get oxygen. Even after parasites are killed, if they are so numerous that they remain clogged in the gills fish will still suffocate.

5) Overmedication burning gills, rupturing blood cells, causing too much mucus production.


Fish may show any unusual symptoms. This should be only offered as a diagnosis after all other possibilities have been ruled out. Erratic, jerky swimming or spinning are common signs of brain damage. Brain damage can be caused by parasites, bruising (concussion), high or low temperatures, or toxins.


Symptoms look the same as brain damage, but all or most of the fish in the system are affected at once. Spinning is the most frequent sign of a toxin. Common toxins are : Windex, ammonia, paint fumes, bug sprays, copper, flea and tick collars, (by hand contact), colognes or perfumes, and chlorine. Water changes is the most effective methods of removing poisons quickly.


These can be caused by:

1) PH that is too high or low.

2) Copper burns.

3) Shipping damage.

4) Scraping on rocks or other objects.

5) Bites.

6) Parasites.

7) Internal infections reaching the outside.

8) Net damage during handling.

Treatment consists of eliminating the source of the problem, and the use of Novaqua, Polyaqua, or any other artificial coating medication. Be sure to watch for signs of bacteria or fungus and treat accordingly if it appears. Salt in the water can be a very effective prevention of fungus or bacteria infection. Clean water will also retard the growth of infection.

NOTE: Many fish diseases are not treatable - either is no known cure or the cure may jeopardize the fish. Some fish die from internal parasites that are undiagnosed and therefore untreated. Do your best to help your system and it's fish, but remember that we cannot do ever...

Aquatic Gardens

Ponds, Streams, Waterfalls & Fountains:
Volume 1. Design & Construction
Volume 2. Maintenance, Stocking, Examples

V. 1 Print and eBook on Amazon
V. 2 Print and eBook on Amazon

by Robert (Bob) Fenner
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