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Freshwater Fish Disease

By: Jennie Kermode - Updated: 4 Nov 2014 | comments*Discuss
Disease Fish Illness Aquarium Infection

Disease is the thing which every good aquarist most dreads. However hard you work to look after your tank there is always a danger that disease will appear to wound or kill your fish. However, there are a number of basic things you can do to make it less likely.

Disease organisms will always exist within your aquarium, but healthy fish are quite capable of holding them at bay. Disease outbreaks generally occur when fish are stressed. Carefully monitoring your water quality and making sure the water is a consistent temperature will help to reduce stress, as will instituting a natural light cycle and making sure there's plenty of oxygen available for all your tank's inhabitants.

You should also be aware that bullying can leave fish vulnerable to disease, so provide places to hide and be ready to separate incompatible fish if necessary. Don't overfeed your fish, as uneaten food can provide a growth medium for disease organisms, but do keep a look out for individual fish who aren't getting enough food. If a fish has difficulty eating it may become underweight and a health risk (once a disease has infected one fish the risk to your other fish is increased too). If necessary, take fish with such problems into quarantine and give them extra food there until they recover the energy they need to look after themselves properly in the main aquarium.

Types of Fish Diseases

The majority of disease outbreaks in freshwater aquariums are caused by the following:

  • Topical bacterial infections -These include fin rot, ulcers, pop eye and gill disease. They're easy to spot because they immediately cause damage to the outside of an infected fish's body, but they can progress fast and require prompt treatment with an antibiotic. They may leave survivors with lasting wounds. You should be aware that wounded fish may later be subject to bullying. Steroid-based treatments like Melafix can help some wounds to heal.

  • Internal bacterial infections -These can be difficult to detect. Often the only noticeable change is in the fish's behaviour. You should consider them a possibility if your fish become listless or suddenly grow darker in colour, if their bodies swell, or if they die without any apparent symptoms. If this is the case, you should intervene immediately with a strong antibiotic.

  • Fungal infections -These affect the skin and gills of fish, especially after injury, and they may occur as a secondary infection following another illness. Antibiotics can help fish to fight off fungus in some cases, but often the most effective treatment is methylene blue, which can be used either as a bath (for quick dips) or as a systemic water treatment.

  • Parasites - Only very occasionally large enough to be seen directly, these are often detectable only through the affected fish's behaviour - flicking and rubbing to try and dislodge them. Some cause skin wounds or cysts, as in the well known 'white spot' disease. They can usually be treated with malachite green. However, in the case of parasites which form cysts, you'll need to dose this carefully to catch them during the free-swimming stage of their life cycle.


Although it is sometimes necessary to treat the whole aquarium during a severe outbreak of disease, it is generally better to treat sick fish separately in a quarantine tank. This avoids exposing other fish to chemicals which may place them under stress and it means you don't have to worry about certain treatments being toxic to plants and to any invertebrates you may have in your tank. Furthermore, quarantining fish reduces the stress to which they are subjected. It means they don't have to compete for food and than they're safe from bullying - the natural instinct of healthy fish being to drive a sick fish away from them.

A quarantine tank can be set up quickly and easily using a simple corner filter unit, a bit of gravel and water from the main tank, and a small heater as required. If your fish can cope with it, it can be a good idea to reduce the temperature of water in the quarantine tank slightly to provide the fish there with more oxygen and slow down the life cycle of the disease organisms.

Fish disease can happen very suddenly and can be a daunting prospect, but, if you catch them in time, the good news is that most common diseases can be treated effectively. As always, the key is to pay close attention to your aquarium and to the general health and happiness of your fish.

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Share Your Story, Join the Discussion or Seek Advice..
@Mark - it's a tricky one to answer with little information, one reason could be water quality as they are very sensitive. Is it a new tank, have you had the fish for a while, have you changed the water chemistry in the tank? Questions like that would need to be explored first before any advice could be given.
Matthew - 4-Nov-14 @ 10:36 AM
I have a Pearl Gourami that is stable in the water but is vertical in the water and is constantly swimming up. Is it a swim bladder problem?
Mark - 3-Nov-14 @ 7:52 AM
This makes raising fry so much easier, thanks for sharing the good practices. I am designing a nursery now, and will incorporate your ideas.
Oldmills Tilapia Far - 4-Jan-12 @ 11:51 PM
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