From identifying the problem to sorting out a solution, check out our guide to fish health.

When it comes to fish health, prevention is better than cure. The majority of fish illnesses can be linked to poor water quality so, if your fish aren’t happy swimmers, start by investigating the water parameters to rule that out as an issue. Partial water changes also go a long way to quickly sorting out problems for fish, and ensuring they are having a well-balanced diet suitable for their species.

However, there are times your fish will have issues and, as a pet parent, it’s up to you to care for them. This guide will assist you in achieving optimal health for your fish friend.

How do I identify a sick fish?

Observation is key. Look for ‘normal’ behaviour for the species you have. The amazing behavioural diversity shown by different fish species is part of what makes aquarium keeping so interesting. Fish that swim normally, act normally, eat well, and have good colour and appearance in general are probably healthy. Behavioural changes may be symptoms of stress, water problems or disease. They can be natural or temporary, such as when fish begin spawning.

When observing your fish, here are some things to look for:

Clamped fins

Fish have clamped fins when they hold their fins closer to their body most of the time, rather than opening the fins in a typical way for that species. The fins may be frayed (torn or damaged) or the fins may be fine, but closed. Clamped fins may be the first indication that a fish is not healthy, because its behaviour has changed. Sometimes other symptoms accompany clamped fins such as ‘shimmies’.


The fish seems to be swimming in place – swimming without moving forward. The fins are often clamped, too. Shimmies are often seen in Livebearers, like Mollies or Swordtails. Cool temperatures or a temperature chill often causes shimmies. Good water conditions and treatments often make this symptom disappear.


You may notice a fish that is swimming in an unusual way. It may bump into an object or along the gravel repeatedly. The fish is attempting to ‘scratch’ itself. This is called flashing. Rare occurrences are probably not cause for alarm. Lots of flashing in your aquarium or pond may indicate external parasites.

Schooling behaviour

Many aquarium fish are ‘schooling’ fish: they prefer to swim with a group of their own species. Healthy schooling behaviour is more easily observed in larger aquariums where the fish have plenty of space to swim. Watch closely – if one or two are not swimming with the main school, check them for other symptoms. Remember that in small aquariums, natural schooling behaviour might be limited.

Swimming behaviour

Many species tend to prefer swimming in a certain level in the aquarium. Danios normally dart about near the water surface. Angelfish tend to swim throughout the aquarium but may have a favourite corner or area where they rest. Obviously, if a fish changes the way it swims, you should investigate. Consider where they swim as well as how they swim.

How can I treat my sick fish?

Check your water first. Poor water quality is the cause of 90 per cent of fish disease. Living in unsuitable water leads to stress and environmental disease and these diseases can look just like parasitic or bacterial disease. Poor water quality also suppresses the immune system of your fish, reducing their ability to fight off disease. Make sure all your parameters are correct and rule this out as a potential issue.

The best treatment is prevention

  • Always quarantine new fish
  • When buying new fish, observe the fish before purchasing
  • Maintain water quality
  • Avoid acute exposure to ammonia, nitrite and nitrate
  • Keep temperature consistent
  • Keep pH consistent
  • Practise good nutrition: provide a wide range of nutrients to maintain health

Other tips to help sick fish

Aquarium salt: Aquarium salt can be therapeutic for fish and can help soothe the symptoms of infection.
Temperature adjustment: The progression of the life cycle of many external parasites can be expedited by raising temperatures.
Medications: Medications can be used to treat specific illnesses. Use medications with caution and be mindful that many treatments will not be safe for plants or invertebrates so choose appropriately.
Act early: Don’t let the problem linger or it may spread to other fish.
Check first: Do not medicate until you have checked your water parameters and eliminated environmental causes.
Clean up: Clean the filter and do a partial water change before treatment, as this will improve fish health.
Remove activated carbon before treating: Activated carbon will remove the medication from the water.

If in doubt, consult your local Greencross Vets. You can also chat with your local Petbarn fish expert in store.


Find out more about maintaining your aquarium here: