Behavioral Signs of Disease in Fish

Behavioral signs of disease in fish are harder to spot for most owners than physical signs of disease. By watching your fishes’ behavior consistently, for at least 5-10 minutes per day, you will be able to determine what is normal or abnormal for your fish. Some “fun fish quirks” may actually be a sign of a serious disease.

Swimming Behavior

All fish should swim level nose to tail and left to right. Fish that swim with a list or tilt are not normal. Some fish may have developmental abnormalities or healed trauma, such as a missing fin, which causing them to swim abnormally. Usually, fish swimming with a list or tilt will need to be evaluated by an aquatic veterinarian to assess the cause.

You may notice your fish avoiding certain areas of your pond or tank. Depending on your system setup, flow and any decor, you may have dead spots in your pond that your fish will avoid due to low oxygen load.

Your fish might not swim swim much further than the immediate area around your waterfall. This can be hard to tune into unless you watch your fish for awhile. They may leave the area for food or not. This can be an indication that your water doesn’t have enough oxygen or your pH is starting to crash.


Most pet fish love to eat. Some fish may have a bigger appetite than others, especially breeding, pregnant females. If you have a fish that doesn’t want to eat, assess if they are able to get at their food. Is there too much competition? Are you feeding an appropriate diet? Is your food old, stale or moldy?

Some fish may have an off day or two and return to their normal appetite. Usually, this is nothing to be too concerned about unless your fish doesn’t eat for 3 days or more. You don’t want to wait too long to get your fish help or they may not have enough energy to recover. It is also worth an assessment if your fish has repeated sessions of 1 to 2 days of not eating over weeks to months.


Neutral buoyancy is critical for most fish. If they float too much or sink too much, they can become stressed and have secondary issues. Some fish, like koi and goldfish, are physostomous fish, with a duct between their swim bladder and esophagus. Too much vigorous surface eating can lead to floating post-meal. Other species are physoclistous, and remove air from their blood stream to inflate their swim bladder. These fish are more prone to buoyancy disorders with water chemistry problems, especially those involving the pH of their water.

And, no, your fish is not constipated and peas don’t have any additional fiber than regular fish food. They work because they sink.


One of the easiest behavioral clues for fish owners to pick up on is a behavior known as flashing. Since fish do not have fingers, arms or legs to scratch themselves, they will use their environment to itch themselves. They may use the bottom or sides of their system, any protruding filtration/plumbing or decorative items.

Often, owners will only see the after effects of flashing. This may include missing scales, bruising or traumatic wounds. Although often caused by parasites, sometimes, another primary stressor has knocked out the fishes’ immune system, giving the parasites entrance to the fish. Again, like other behavioral signs of disease in fish, you must take some time to watch your fish in order to note some of the more subtle signs of disease.

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