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Back To Fish
Fish Health 101

Fish Health 101

Sickness in an aquarium is not uncommon but predominantly occurs when water conditions are poor, or in times of stress or changes for the aquarium.


The majority of sickness can be prevented by regular water testing, frequent partial water changes, careful observation, and appropriate living conditions. Carefully observing your tank for any early signs of illness is essential to get a head start on treatment.

Water quality and environmental changes

Poor water conditions can cause fish to easily become stressed and susceptible to infection.

Many illnesses can have overarching water quality concerns, and an initial wound or scratch may develop into secondary bacterial infections or fin rot so it’s not always clear what the root issue is. Water quality issues can have a flow on effect several weeks after the issue has been remedied.

Regular water testing can tell you a lot about the health of your tank, and come in assistance when trying to identify the cause of an illness.

All fish keepers should have a basic test kit to check for PH, KH, Ammonia, Nitrite and Nitrate. These tests will help tell you about the current cycle in your tank, and any concerns that may require attention (such as a dipping PH) for more information about cycling your tank click here.

Disease outbreaks are relatively uncommon in maintained aquariums with little change to the day to day life of the fish. However any change to the environment can bring added stress and risk and while this won’t always result in sick fish it’s important to understand how these changes could potentially affect your tank.

Changes that can affect the tank, filtration and its inhabitants may include:

Moving house

  • Moving house and setting up your tank again often involves removing all water, filtration, and transporting the fish and aquarium into a new space before having to set it all up again. Much like setting up a brand new tank.
  • It’s not unusual for a tank to ‘re-cycle’ or go through issues with its nitrogen cycle due to the large water change and potential damage to the delicate biological filtration.

Setting up a new aquarium/Cycling your aquarium

  • New tanks are more likely to have issues with the balance of the nitrogen cycle and do require additional support and gradual stocking of fish while they stabilize.
  • You can read our articles on "Setting up your new aquarium" and "The Nitrogen Cycle" for more information.

Changing décor

  • Adding in new ornaments or plants is a flurry of activity into the tank while you change up your décor, and is often paired with a water change. This can be stressful for the fish that additionally have to get used to a new territory and sort out any squabbles for space.

Large water changes

  • Adding a large volume of new water that may be different to the water within (keep an eye on the readings of your tap water as these can be subject to change, and a rested stable aquarium may slowly differ from the norm of your tap water over time)

Filter and Filtration Cleans

  • Whenever cleaning the filter, it is important to retain as much of the original media as possible and change one section (e.g. just the filter wool, or just the sponge, or a small portion of bio media) at a time to allow for the good bacteria to recolonize

Adding new fish

  • Bullying, competition for food and territory, potential addition of disease, parasites, stress of arriving for the newbies

Seasonal changes

  • Temperature, affected oxygen levels in the tank

Preventing the spread of illness

The best way to reduce illness in your aquarium is to monitor for early warning signs, and to not introduce sick or at-risk fish to the tank.

New arrivals

When bringing home new fish it’s recommended to quarantine these fish for 2-3 weeks before acclimating them into the main aquarium.

Isolating newly purchased fish helps assess their health and prevent spread of any underlying disease or parasites before they risk your other fish. This also allows your new arrivals time to settle after the stress of transport. New fish are at the highest risk of illness, so giving them time to settle will further boost their immune system and natural ability to fight off infection, and ultimately better prepare them for joining the main group where there is additional competition for food and space.

Quarantine tanks can be a small simple tank with a basic sponge filter (and a heater if needed for tropical fish). Sometimes it can be a good idea to dose with a prophylactic supportive treatment during this time, such as API Melafix and Pimafix which can help repair slime coat damage and cover any minor bacterial or fungal issues that may arise.

It’s important to recognize that all fish will carry some level of pathogens and parasites, and even healthy looking fish when purchased can be at risk of harbouring unknown issues.

Parasites can go undetected and of no consequence to the fish unless their immune system becomes compromised and at this time the parasite may spread and become visually apparent as it moves into a new life cycle.


Hospitalise unwell fish

If you have identified that there are sick fish in your tank, it’s important to further isolate these fish into a Quarantine tank. This helps prevent the spread of any illness within the tank, and allows for proper assessment of the issue and the affected fish.

A smaller environment can be more accurate for medicating, uses less vital medication, and also will save your biological filter media and plants from some of the potential effects of some medications.

In some instances the whole aquarium may need to be treated as well, while some individuals may still need to be split off into an isolation tank.

Treating the affected fish separately also reduces the risk of the medication negatively affecting non-target species, as certain medications can be dangerous to species of aquatic fish and invertebrates (such as snails, loaches, knife fish etc.).


Have the right setup

Aquariums are the lifetime home for your fish, and they eat, breathe and live in the water surrounding them.

Ensure your setup is best suited to the number of fish, and types of fish that you are looking to keep. Assess correct tank size, compatible tank mates and numbers of tank mates, appropriate filtration and flow of water for the fish, and the right varieties of food to ensure their needs are being met. 

The key to preventing illness is early intervention, so keep an eye on your fish for any changes in behaviour or condition and regularly water test your aquarium and tap water.

Steps to diagnosing illness 

  1. Quick intervention is crucial when it comes to illness and health care. When you meet a health issue with your fish there are some key steps to take:
    • Does anything require immediate action?
    • Are there dead fish that need to be removed or unwell fish that are high risk and need to be moved to a quarantine tank or to an isolation floating container?
  2. Assess the situation, what is the issue?
    • Illness or Injury (white fins, wounds, red gills, fungus)
    • Body changes (swollen belly, unusual lumps)
    • Behaviour (are there fish in distress; gasping at the surface, unable to swim etc.)
    • Setup ( e.g. are the filters, heater, air pump all functioning)
    • Other changes (have you just added something to the tank – hands, treatments or foods to the tank)
  3. Prevent further illness or concern
    • The assessment of the issue impacts your next steps.
    • It’s always a good idea to conduct a water test at this stage which will give you a quick screenshot of the current situation and if the water may be impacting this. Mechanical issues such as filters becoming unplugged are also likely to affect the water quality.
    • Remove any additional fish to a quarantine tank that will require treatment
  4. Administer treatment or appropriate fix if requires
    • Once you’ve checked your water test and assessed the condition of your fish, you will be in a better place to decide on a medication course or seek further help if required.
    • Not all illnesses require treatment, some fish develop age or genetic bumps and lumps that normal treatment cannot fix, and is not often contagious to other fish.
  5. Do a water change. The majority of sick fish will get benefit from a water change, especially if you need time to assess a larger treatment plan this will give you a head start.


Always ask for help if you are unsure of how to proceed, as properly diagnosing an illness can be difficult even for the most experienced aquarist. It can be tempting to try to fix issues with drastic action in a desperate bid to save your fish.  (i.e. doing large water changes, scrubbing the filter and adding multiple medications and treatments) however you should never need to ‘strip’ your tank of all water and filtration at this point, so keep a clear head and assess the issue.

If in doubt, remove a cup of water to test (all Animates stores offer free water testing services) and then conduct a small water change (around 20-30%). If your fish are obviously gasping at the surface/showing difficulty breathing, conduct a slightly larger water change.

Many illnesses if caught earlier can be alleviated by improving conditions, water changes and monitoring rather than intensive medications.


Treatment basics

New Zealand has limited aquarium medications available compared to the international fishkeeping hobby, as such, managing the water quality and general wellbeing of your fish remain the best action to prevent any illness outbreak.

When treatment is required there are several broad-spectrum solutions that can be used in conjunction with regular water changes to help your fish recover.

There is only one type of antibiotic available for purchase in stores; any other antibiotics required to treat your aquarium will need to be prescribed by a veterinarian. It’s important to minimise any potential overuse of antibiotics as there are risks of developing resistant pathogens. If completing an antibiotic course be certain to complete the full treatment according to the instructions. Antibiotics can also damage your filtration, so make sure to add in additional live bacteria supplementation to support the recovery of the biological media.

If there are multiple issues that require treatment it’s best to assess the highest need or root cause for treatment, and only treat with one medical solution at a time. Some issues are secondary to the root concern (i.e. secondary bacteria infections from wounds – the wound may have healed with clean water changes alone, but the secondary infection may require additional support).


General medication tips

  • Always follow the dosing instructions on any product label.
  • Remove your filter carbon prior to treating and turn off UV sterilizers.
  • Do not use more than one disease treatment at a time (exception is aquarium salt with other treatments).
  • Keep all treatments out of reach of children.
  • Wear gloves and safety glasses when handling medication.
  • Invest in a basic quarantine tank.
  • If in doubt, ask for help. The team at your local Animates store are more than happy to help if you have any concerns.


Common illnesses and concerns

There are a wealth of issues that may arise and thankfully, most fish keepers will never come across the majority of them. Each issue has a differing treatment plan and medication, so always adhere to instruction labels and seek further advice if required.

Look out for any changes in swimming behaviour, resting or activity patterns as well as obvious health issues.

For more detailed information and treatment plans please discuss with your local Animates store for specific medications and treatment details.


Illnesses and symptoms

White Spot/ich (Parasitic)

  • White spot is an aquatic parasite that increases its presence when fish are stressed. It can be fatal in many instances. White Spot resembles grains of salt on the skin, sometimes raised. Affected fish will often scratch against objects, have clamped fins, and may gasp at the surface or rest on the bottom of the tank. 

Ammonia poisoning (Other)

  • Poisoning due to a high level of ammonia. Ammonia irritates the gills of the fish making it hard to breathe and absorb oxygen. Fish will often gasp at the surface and have reddened gills, highly dangerous and can kill large numbers of fish very quickly. Is commonly seen in uncycled and new tanks setups. 

Fungal infections (Other)

  • Grey or whitish growths or ‘fluff’ on the skin and fins, in severe cases the fins may show signs of Fin Rot as well. Fish who develop fungus are often already in a weakened state and it can be the result of other health issues or injuries. Fungal infections are often confused with bacterial infections.

Dropsy (Bacterial)

  • The first sign of dropsy is a heavily swollen body, in severe cases the fish can ‘pine cone’ and have protruding scales and redness. Dropsy is a bacterial infection of the kidneys causing fluid retention and renal failure. It’s often fatal and is more commonly seen in pot-bellied species such as Fantail or fancy goldish, Gouramis.

Haemorrhagic Septicaemia (Viral)

  • An infection caused by fish already infected with the virus VHSV. Not commonly seen, however it causes haemorrhaging throughout the body and organs, infection of the kidneys causing fluid retention and renal failure.  Some fish exhibit no symptoms but key signs are bloody looking red patches across the body, open sores, lethargy, bulging eyes, and bloated body.

Lice  (Parasitic)

  • A physical attachment of a live parasite (the louse) to the fish, they pierce the skin and feed on the host. Lice can often be seed as moving dark specks and in some instances are visible as small minuscular crab like structures crawling on the fish.

Fin and Tail Rot (bacterial)

  • White edged or translucent tail and fins, varying states of decay, fraying and ‘rotting’ of the fins. Fish often show a loss of appetite and are extremely listless. Finrot is often a secondary bacterial infection after some sort of injury from bullying, décor or poor water quality.

Columinaris (bacterial)

  • A highly contagious bacterial disease causes by gram negative bacteria. Often fatal and spreads quickly. Can be confused with a fungal infection because of the white or greyish white spots on the head but these are usually paler and may become yellow/brownish in colour and be tinged red at the edges.
  • The mouth is normally attacked by these bacteria, although lesions can appear on the back extending down both sides of the body giving the appearance of a saddle.

Swim Bladder Disorder (other)

  • The swim bladder of a fish is responsible for maintaining buoyancy. The cause of SBD can be constipation or bloating due to gulping at the surface by feeding. In some cases there is an underlying bacterial infection or organ malfunction. Fish with SBD struggle to maintain a normal position and will spin or swim abnormally, floating belly up and tail high. This disorder mainly affects goldfish and bettas, or fish with a heavy set midsection. 

Velvet (Parasitic)

  • A miniscule parasite, velvet infestations often show as a fine gold film or spotting (smaller than white spot) as if the fish is covered in a layer of dust. Velvet is likely caused by abrupt changes to water temperature, poor diet or water quality. 

Anorexia (Other)

  • A skinny fish is often a sign of an underlying issue. It can be symptomatic signs of stress, parasites or other ill health. Thin fish should be checked to ensure they are being fed an appropriate diet and aren’t being bullied or outcompeted at mealtimes.  

Clamped fins (Other)

  • Fish can be symptomatic of several issues and often the fish have severely reduced activity and appear hunched and quiet. Clamping of fins is often one of the initial signs of a stressed, unhappy fish who may be weakened or more susceptible to further sickness or is being bullied.

When in doubt – take a sample of water for testing and complete a pre-emptive water change. All Animates stores offer free water testing services for all customers.