How to Tell when Fish are Unwell

Published on April 27, 2023

Trout with Scoliosis
Scoliosis in trout is often caused by nutritional deficiency from old or poor-quality feed.

Even though they can’t talk, once you are around fish for a while, you begin to see how they communicate with you. When I managed fish hatcheries, I was amazed how quickly you could look at a tank of fish and immediately get an idea how that population was doing. Our pond and aquarium fish are our pets. We get to know individuals very well and learn how different each fish can be so that we can recognize normal behaviors and appearances. 

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Once we have learned what normal is, we’ll be able to recognize when a problem comes along. After a long conversation with our fish, we can build a list of clinical signs, such as behavioral any physical changes, that will point us in the right direction toward solving the problem. These visual conversations with our fish are critical to catching something quickly and saving our fish’s life.

Abnormal Fish Behaviors

When fish are not well, they will often slow down. Lethargy is usually one of the first behaviors we notice, and it is common with many different problems. For example, fish become sluggish when they cannot get enough oxygen from the water, either because there isn’t enough oxygen going into the water, temperatures are outside the fish’s tolerance zone, or the fish are having problems with their gills. Infections from parasites and bacteria can also impact fish gills, along with the rest of the body, causing them to slow down. When your fish show lethargy, it is good to take a closer look to see what may be going on.

koi with pine coning
Ascites, dropsy and pine coning in koi are often caused by viruses and bacteria.

Heavy breathing is often the next phase of fish having difficulty getting oxygen from the water. You will see rapid mouth movements and exaggerated movement of the gill covers. Gradually dropping oxygen levels will cause heavy breathing and really warrants a closer look at the fish and the aeration system. Pathogens can cause gill inflammation, and as this progresses, breathing will also become heavier. High levels of nitrite cause brown blood disease, which prevents the hemoglobin in the blood from carrying oxygen.

When there is a lack of oxygen in the water, as seen with aerator failures, gasping at the surface will come on quickly and can kill quickly, often in minutes to hours depending on density. If the gill damage is restricting the fish from extracting oxygen from the water, surface gasping will usually come on more slowly, often over days.

Unhappy fish will often move with their fins closed. Like lethargic behavior, clamped fins are often an early sign of an issue. It can also be one of the first signs of a parasitic infection.

Fish not feeling well will often seek out specific locations where they feel better. Those with oxygen issues may position themselves near waterfalls or water jets where the water will have the most oxygen. Lethargic fish often seek out areas with little current so they don’t get pushed around and have to swim. You may also see fish hiding more than normal, which may be because of a disease or from constant harassment from a predatory bird, animal or another fish.

Lying on the bottom is often the next stage from searching out a quiet spot. If there are lots of structure where fish can hide, it can be difficult to find them at this stage. That’s why it is really important to recognize changes in fish behavior while you can still find them. When on the bottom, it is also very difficult to inspect the fish for physical problems, such as ulcers.

Fish that have an irritation on the skin will often scratch on structures in the pond. Not all scratching is abnormal. Healthy fish will scratch now and then. You should recognize what normal scratching looks like and become more attentive when you see increases in frequency. Increased scratching most often indicates a parasitic infection, but it can also occur with increased ammonia levels or chemicals that irritate the skin. With heavy scratching, fish may create lesions and risk secondary bacterial infections that can become bleeding ulcers.

If fish are rolling, spiral-swimming or otherwise swimming erratically, there are usually major issues. Swimming like this is often associated with neurological issues. These can come from viruses and bacteria but also from a prolonged exposure to low oxygen levels or a toxic chemical that got in the water. The most common chemicals are chlorine and chloramine from tap water that wasn’t properly treated. Overzealous fish may also jump out of the water and smack their heads, causing erratic swimming. Depending on the severity the symptoms, it may subside on their own.

Sensitivity to Stimuli

If you have an aquarium and tap the glass, you will notice how the fish react. Different species react differently, but they usually will react. The same can be observed in your pond fish. If something drops in the water, the fish will often jump or skitter off. Fish that are not well will respond differently to stimuli. Lethargic fish may barely respond, while fish that are irritated by something may be extra jumpy. Pesky predators can make the fish much jumpier.

A change in feeding behavior can be an excellent early indicator of a problem, and warrants a closer look to see what may be going on. Frequent predator visits can put fish off feed and cause an increase in jumpiness along with oncoming infections. Poor-quality feed may also promote reduced feeding.

When fish have a skin irritant, they will often produce more mucus than normal. When this is released in the water, foam will be created at waterfalls or by bubbles from air stones. If you see above-normal foam production, look for a parasitic infection or some other irritant like a chemical.

Physical Condition of Unwell Fish

Fish not feeling well will often go dark in color. Stressed fish may also look darker in color, indicating something is wrong. The stress can be from a disease, but it can also be from other sources, like predators and chemicals.

Fish may appear lighter in color from temporary stress, such as from handling, but their color will return when the stress is removed. A lighter or grey color that persists without other stress is most often associated with a thickening of the skin. This is most often from protozoa parasites, but it can also be from an irritant in the water. Both parasites and irritants cause an excess production of mucus which adds to the lighter color.

When fish are grayish or lighter in color, they also may look duller than normal. Healthy fish look shiny when the sun shines on them. With the inflammation of the skin and the extra mucus, that nice shiny look can go away.

You may see cloudy eyes when the fish are looking lighter or gray in color. The inflammation of the skin can make the eyes look whitish. Other physical causes include sunburn and scratching on abrasive surfaces like concrete.

Visible Lesions and Sores on Fish

how to tell fish are unwell -fish with lesions
Ulcerative lesions in koi (left) are often due to the bacteria Aeromonas hydophila. Oral lesions on the mouth (middle) and tail (right) of white sea bass can be caused by Cytophaga bacteria.

Popeye disease, or exophthalmia, is pretty rare in the pond. It is caused mainly by bacterial infections or nitrogen supersaturation in the water. By the time you see this symptom, the problem has usually progressed and may be hard to reverse. Koi Herpes Virus (KHV) can have the opposite effect, where the eyes become sunken into the head.

We examine fish egg samples for spawning readiness.

Changes in gills are much more difficult to see. You would have to handle the fish and open the gill cover to see what is going on. If the fish are struggling, it may be worth handling one fish to take a peek. If gills are abnormal, swollen, red or eroded, the issue is most likely bacterial and parasitic in origin. Brown-looking gills, however, are from high nitrite levels. The dreaded KHV will produce white patches on the gills.

Scale loss can occur for various reasons. A new ulcer will have scale loss prior to eruption and may also have some bleeding associated with it. Predators can cause scale loss, usually on the head or back. Scales can also be lost from aggressive scratching on abrasive surfaces like concrete.

A distended abdomen is usually associated with organs shutting down and fluid accumulating in the abdomen. This is accompanied by “pine coning,” or protruding scales. The most common cause is fish tuberculosis bacteria, which is unfortunately untreatable. However, distended abdomen can also be from a viral infection. This is most often seen in one or just a few individuals. Individuals with distended abdomens should be removed from the population.

Lesions on a fish’s body come in many shapes, sizes, and appearance. Where they occur on the body is also important. Large, bloody lesions are often caused by a bacterial infection such as Aeromonas. White spots like salt on the fish are caused by Ich. Wounds on the back and head are often from predators. Take note of what lesions are normal for your fish.

Fin erosion and fraying can occur from aggressive scratching on abrasive surfaces. Bacterial infection can erode fins edges often with white tufts. Fin nipping can occur from other animals such as turtles or aggressive fish. Don’t forget about predators possibly affecting the fish.

Other Questions to Ask

How many fish are affected? One or many? Is there any mortality, and if so, is it occurring rapidly or slowly?

If we are listening to our fish, we can often nip a problem in the bud and prevent a serious issue. Clinical signs associated with parasitic infections are easy to understand. If we catch it early, treatment can usually resolve the infection relatively quickly.

Sometimes it is not so simple, and we need a longer conversation with our fish to figure out what is going on. In these cases, we may be dealing with external factors such as toxicity from overspray of a pesticide, or nitrogen supersaturation caused by an air leak from a loose or cracked fitting on the intake side of the pump. These problems are harder to diagnose and require more in-depth conversations with the fish and their habitat.

As you can see, these conversations yield valuable clues to help you or a fish health professional solve the problem. Never stop having conversations with your fish. It can save lives and anguish. Read more articles on fish health.

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