How to Tell if Your Fish Are Under the Weather

How to Tell if Your Fish Are Under the Weather

Fish help bring a pond to life.  They’re beautiful and relaxing to watch.  Koi especially are very friendly and recognizes faces and routines.  Once they get comfortable in their surroundings, koi and other pond fish may swim to the surface and say “hi” when you walk out to feed them.  Koi are pretty smart and can be even be trained to do little tricks for added fun.

Fish are great for kids, they’re outdoor pets that require less attention and maintenance than a puppy or kitten.  They can help teach responsibility, as they take turns feeding and even helping perform water quality checks.  Just like any other pet, they can get sick or injured and can require special care.

Your local vet probably isn’t equipped to treat a sick fish.  Luckily many koi and other fish illnesses can be linked to water quality issues or fixed with an over-the-counter treatment from your local fish or pet store.  Daily trips to your pond for feeding or relaxing are opportunities to get to know your fish better.  Many symptoms have visual clues you’ll notice if you pay attention and know what to look for.  When you feed your fish every day, try to observe your fish, keeping a lookout for any abnormal behaviors in swimming patterns or feeding habits.  Look at each fish individually for a few seconds to see if there’s any spots, lumps, scrapes, wounds, or different patches of colors or bumps.  The sooner you catch an illness, the better off your fish will be.

Pond fish illnesses tend to fall into four categories: viral, bacterial, fungal, or parasitic infections.  Pond fish can get sick for many reasons, the most common is water quality, which can create a breeding ground for parasites, fungi, viruses, and bacteria.  If you notice any odd behavior or bumps, always start with a water quality check.  Poor oxygenation, unbalanced pH, and high levels of contaminants, like ammonia or nitrates, can cause stress and sometimes death if not treated in time.  Water quality can drop due to overcrowding, which can lead to oxygenation and excess waste.  The stress from poor water quality can lower your fish’s immune system and put them at risk for infection.

If only one or two fish are sick, then isolate and treat them separately so the illness doesn’t spread.  If multiple fish are sick and it seemed to happen overnight, the issue probably lies in the water quality.  Parasites and other pathogens tend to work more slowly, affecting one fish at a time.  


Behavioral signs:
  • Gasping at the surface: this can be a sign of poor oxygenation in the water.
  • Jumping in the air: chances are your fish aren’t “jumping for joy” to see you, especially koi and goldfish.  Jumping can be a sign of poor quality quality, including overcrowding and poor oxygenation.
  • Congregating at the surface of the pond: this may be normal during feeding time, but if they are doing this all of the time, it could be a sign of an oxygen problem.
  • Flashing: if your koi are swimming quickly and then rubbing up on rocks or the sides/bottoms of your pond, they’re probably trying to remove parasites or are stressed.
  • Other signs:
    • Swimming erratically ex. upside down, in a strange pattern
    • Lethargia
    • No appetite
    • Hanging out by the bottom of the pond
    • Fins held tight against body


Physical symptoms:

Koi can be multicolored, with patches of different colors, but it’s important to look closely for anything out of the ordinary on their scales and bodies.  Scrapes can be signs of parasites, a bully fish, or even an attack from a predator.  Be on the lookout for any white, black, or red spots.  These can be signs of different infections and fungi.

  • Fin appears to be “rotting away” on the edges, possibly white and streaked with blood:   usually a bacterial infection called “fin rot”
  • Dulling color: could be a cold or a more serious infection
  • Any wounds or red sores: could be a parasite like anchor worm or lice
  • White dots that look like sprinkled salt: usually a parasite called Ich
  • Spots that look like cotton: usually a fungal infection
  • Swollen abdomen, scales that poke out like a pinecone: dropsy, a usually fatal condition caused by bacterial infection or kidney failure
  • Black spots: possibly parasite called a fluke
  • White grayish lumps on koi: usually “Carp pox,” a viral infection
  • Cloudy eyes: usually a bacterial infection
  • Other symptoms:
    • Inflamed or red gills
    • Bulging eyes
    • Weight loss
    • Mucus
    • Increase in slimy coating

If your fish have any of these signs or symptoms, first perform a water quality check to see if there are any issues with the water.  Quickly treat the issue and keep an eye on your fish.  If you believe there is an infection of some sort, go to your local fish/pet store and use an over-the-counter treatment for the issue.  You can quarantine and treat a sick fish, or you can treat the whole pond if multiple fish are ill.


Preventive measures:

Maintaining your water quality is the number one way to help your fish stay healthy and stress-free.  Make sure your pond isn’t too crowded or full of debris.  Overfeeding fish may also lead to poor water quality, so make sure to only feed your fish what they will consume in a few minutes.  If your pond is shallow or if you live in a very sunny and warm climate, you may need to provide a little shade for your fish. Oxygenate your water using an aerator, especially in the warm summer months to ensure sufficient oxygen levels.  Regularly check water quality to make sure levels are where they should be.  You want to treat any issues before your fish start showing symptoms to avoid stress and illness.


You may feel a little helpless if your fish are under the weather.  It’s not the same as wrapping your pet up in a blanket until it feels better.  Luckily there are many over-the-counter remedies for various illnesses your fish can come down with, and a lot of resources online for more help.  Observe and get to know your fish so you will be able to quickly notice any behaviors or symptoms that are out-of-the-ordinary.  Paying a little extra attention during your daily feeds could help you catch an illness or water quality issue before it gets too serious!


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