Diseases of Pond Fish: What You Can Do

Diseases of pond fish

Fish bring life to the garden pond, are beautiful to look at, and can help with algae control and mosquito larvae. You take care of clean water and avoid algae formation by removing impurities from the pond and carefully dosing your fish’s food. Still, at some point, your pond fish may get sick. New fish can introduce diseases, but they can also be caused by improper feeding or poor water values. Learn in this article which fish diseases can threaten your pond fish and how to avoid them.

Causes of Fish Diseases

If the biological balance in your pond is in order, your fish will be healthy. In most cases, fish diseases indicate that the natural balance is disturbed and something is wrong with your pond. In a pond that is too small, the fish cannot move enough. Competition for food is high if the stocking density is too high. This can mean malnutrition for the fish. A pond that is too small and a large stocking of fish can also lead to poor water values, which can cause disease in the fish. The fish’s immune system can be weakened in a too-small pond, as the fish suffer from stress. The wrong food or too little food can also make your fish sick.

If the pH in your pond fluctuates, it can weaken the fish and make them susceptible to disease. However, it is normal for the pH to be lower in the morning than in the evening. Therefore, you should check the pond levels regularly.

If you introduce new fish, diseases may be introduced. This can be the case when you buy fish and when you get fish from other pond owners. To prevent your fish from being infected by the new housemates, you should first place the latest additions in a quarantine tank and observe them for about two weeks. If the animals show no signs of disease, you can put them in the pond.

How to Recognize Diseases

Experienced pond owners can detect diseases in their fish at an early stage. You should therefore devote some time to your fish and observe them. Often you can recognize diseases already by the behavior of the fish:

  • Lying at the bottom of the pond
  • swimming disorders such as standing on their heads or staggering around
  • standing around on the water surface for a long time
  • missing escape reflex
  • lack of appetite
  • rubbing against the bottom or walls of the pond
  • advancing swimming.

If the fish suffer from itching, they occasionally jump out of the water. Altered breathing is often difficult to detect. However, you may notice emergency breathing if the fish are breathing at the water surface.

Often, diseases are externally noticeable with color changes, changes in body shape, emaciation, or coatings on the skin’s surface.

In addition, diseases can have other signs. However, it would help if you reacted quickly when you noticed sick fish.

Sick Fish – What Could be the Treatment

Pathogens can spread quickly in the water and infect the entire fish population. Therefore, there is an acute need for action. Simple remedies such as salt baths or medications from the pet store are often sufficient for treatment. Changing the pond water can also help. However, there is a risk that the water will turn green after a short time, just like a newly created pond. You should add algae killer to the water when you notice a green coloration to avoid this. It is often enough to replace only a more significant portion of the pond water. This will allow the ecosystem to rebuild more quickly.

Some medications used to treat fish diseases are so-called broad-spectrum agents and can work against several diseases. However, there is no panacea. Therefore, you should diagnose your fish as accurately as possible when they become ill. The unnecessary administration of medications severely stresses fish. Resistance to an active ingredient may develop.

If you cannot diagnose a disease, you should consult a veterinarian familiar with fish diseases. He can provide specific treatment.

Common Diseases of Pond Fish

Below you will find an overview of the most important diseases of pond fish and their treatment options. Since there are many different diseases, no claim to completeness can be made here.

Fish Mold

Fish mold is a fungus that affects fish with a weak immune system but hardly healthy fish. The pathogen lurks almost everywhere. Itching, changes in the mucous membrane, and cotton wool-like coatings indicate an infestation. You can treat the disease with higher concentration salt baths or remedies containing malachite green.

Hole Disease

Pitting disease is caused by bacteria. Other pathogens or unsuitable environmental conditions may also be involved in the disease. The fish exhibit ulcer-like holes. Mainly they occur on the trunk and towards the tail fin. The disease progresses rapidly. You should consult a veterinarian specializing in fish who can provide treatment.

Swim Bladder Inflammation

In the case of swim bladder inflammation, the fish can no longer provide buoyancy and can only move with difficulty with their fins at the bottom of the pond. Bacteria, parasites, and metabolic disorders cause the disease. The veterinarian must determine the cause and provide appropriate treatment. Increasing the water temperature to 25 degrees Celsius and adding iodine-free table salt can also help.

Fins rot

Fin rot manifests itself with frayed fins or discoloration at the fin edges. The disease often develops under unfavorable husbandry conditions. You should consult a veterinarian who specializes in fish for treatment.

White Spot Disease

White spots on the mucous membrane are signs of white spot disease. The eyes of the fish may be affected. If the gills are affected, respiratory distress occurs. Single-celled parasites cause the disease. You should treat it with malachite green for at least five days.


Costia is a unicellular parasite that affects only weakened fish. Whitish-blue cloudiness forms on the mucous membrane. Salt baths may help. Since Costia can promote other infections, act quickly and consult a fish veterinarian who will perform a root cause analysis.

Sucking Worms

These tiny worms can make life miserable for fish. They settle on the skin and gills and are barely visible to the naked eye. If the skin is infected, you will recognize this by cloudy coatings and the fish rubbing against the pond’s edge. If the gills are infested, the fish have difficulty breathing. Salt baths or preparations from the pet shop can help.

Carp Louse

The carp louse grows to about 13 millimeters in size and is introduced by waterfowl. It sucks on the skin and scale pockets. It can cause redness and inflammation. You can pick off individual carp lice, while you should use a remedy from the pet store for more severe infestations.

Koi Herpes Virus

An infection with the Koi herpes virus is a notifiable animal disease. It makes itself felt with massive damage to the gills. Respiratory distress occurs. Skin damage can occur. It would be best if you put the sick animals into another tank. Treatment by a veterinarian is necessary.

Carp pox

Carp pox occurs when temperatures drop and are noticeable with waxy, translucent deposits on the skin and fins. They are favored by a weakened immune system and are usually harmless. It would help if you optimized the husbandry conditions.

Energy Deficiency Syndrome

Energy deficiency syndrome often occurs in obese fish and with a deficient diet. The scales of the fish may protrude like pine cones. In some cases, the fish are swollen. They show slow reactions. You should add some salt to the water, increase the water temperature by two degrees Celsius daily and give highly digestible food.

How You can Avoid Fish Diseases

You can mostly avoid fish diseases by optimizing the keeping conditions. You should check the water values regularly, especially in the summer months, and replace part of the water. A water filter and pump and a pond clarifier can improve water quality.

To prevent algae formation, you should regularly remove impurities such as leaves that have fallen into the pond. You should also declare war on your filamentous algae. Make sure that the fish population matches the size of the pond. If fish has increased, you should remove some fish and give them to other pond friends. You should use high-quality feed and adjust the feed to the needs of the fish.

Find a fish veterinarian nearby. Fish veterinarians often offer a check-up for the fish in the spring and fall.

It would help if you cleaned your pond twice a year to prevent disease in the spring and fall. This is where a pond mud vacuum can be a great labor saver.


Fish diseases can occur due to new additions to the fish, but also too small ponds, too large fish stocks, incorrect feeding, or poor water values. If you practice good pond hygiene, you can ensure a biological balance and prevent diseases. You can treat various diseases with simple means such as salt baths. However, many diseases require an examination by a fish veterinarian, who will provide the appropriate treatment.

Florian Egert

I am Florian Egert, the owner of pondlovers.com. I live with my wife and two children in Germany in a small village in the countryside.

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