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November 2023

Sick Fish: Diagnosis and Treatment


Is your fish looking or feeling unwell? Here's some tips on identifying a sick fish, how to prevent disease, and what treatment is available.

When compared to dogs or cats, fish are generally considered low-maintenance pets, but they can still get sick. They are particularly affected by stress, which can be caused by a variety of things like travelling, poor water quality, poor nutrition temperature fluctuations and even over-feeding. In this article, you’ll find helpful information around caring for and recognising when your fish is sick, because prevention is always better than cure!

Please use this as a guide only. If you think your fish are sick, please reach out to your vet or feel free to speak to your friendly local Petstock team. Find out where your nearest Petstock store is with our Store Finder.

Caring for your fish

Providing consistent, proper care for your fish will go a long way in prevention. To understand how to best tend to them, you should first get to know them better:

  • Learn about the fish you buy: Do your research on the species and their requirements, particularly around things like tank size, preferred water temperature, food and equipment needed.
  • Observe their behaviour: By regularly observing your fish, you can start understanding what normal behaviour is, so you will know when something is off.

Fish are very susceptible to stress, which can then weaken their immune system. To ensure you're providing a low-stress environment for your underwater friends, all you need is regular maintenance and care. Here are some tips to start you off:

  • Clean and maintain your fish tank regularly: We recommend changing the water every 2 weeks (though this depends on your tank too), keeping the tank free of algae and maintaining the tank filter. For more details, check out our PET Smarts guide to How to clean a fish tank.
  • Feed your fish what they need: Over-feeding not only dirties the water but can also cause your fish to be sick. Additionally, ensure that you're providing a nutritious and varied diet.
  • Ensure the tank is big enough: You’ll want to give your fish room to swim around!
  • Ensure your fish are compatible: Before you buy, do your research on the different species of fish to make sure you’re buying ones that are compatible and can live peacefully with each other.
  • Monitor and maintain the water temperature: Particularly if you’ve got tropical fish, as they need warmer waters. Check with your local PETstock team on the optimum temperature for your fish tank.
  • Test your water regularly: A lot of the stress associated with illness can start with incorrect water parameters.  By testing regularly, you can prevent stress and illness rather than having to look for a cure.

Recognising when your fish is sick

If you notice that your fish is behaving abnormally, seeming distressed, or hiding or not eating as per usual, these are symptoms of stress. First, always check on the things you can control - is the tank clean and well-maintained? Are there any aggressive fish in the tank too? Is the temperature correct?  Remove whatever stressors you can find and if your fish still seems stressed, it could very well be sick.

Take a good look at your fish and try to find symptoms of diseases. Fish can get both fungal and bacterial infections, and some of the symptoms of disease include:

  • Loss of appetite, or regurgitation of food eaten previously
  • Lethargy, lack of active swimming; fish that are sick may stay at the bottom of the tank for long periods of time
  • Change of colours into duller shades, even grey
  • Scratches, wounds, open sores that don’t seem to heal
  • White patches, lumps or spots on the fish
  • Tails or fins seem clumped up, stiff, even falling apart

Some common illnesses that fish suffer from include:

  • White Spot/Ich – appear as raised white spots on a fish’s skin or fins
  • Fungus – there are a couple of types of fungus, but mostly appear as fluffy white growths on the body of a fish
  • Fin Rot – appears as ragged or frayed fins
  • Gill Mites – appear as tiny critters attached to a fish’s gills
  • Lice – appear as small critters or crab like creatures on the fish’s body
  • Dropsy – appear as swelling on a fish’s abdomen

If you think you’ve spotted one or more of the above symptoms, we recommend you quarantine the sick fish to avoid getting the other tank mates sick. Then, if possible, take some photos of videos of the symptoms and bring them into your local Petstock store or vet to check if anything can be identified in this way. It's best to avoid transporting the fish; as this can cause stress. However, if you are very concerned about your fish, do contact your local vet for advice.

If you’d like more detailed advice on pet care, stop by your local PETstock store and chat with our friendly staff.