You need to be mindful of health risks when owning fish, but with our guide to fish diseases and infections, you’ll know the symptoms and how to treat them.

Poor health in fish isn’t uncommon, but the main cause is poor water conditions. Healthy water usually means healthy fish, so paying close attention to the condition of your tank is vital. Remember that you can always bring in a sample of your aquarium water to your local Petbarn and have it tested for free.

Here’s our guide to some common diseases and infections that can affect fish and the best ways to treat them.

Ammonia poisoning

Though not a disease, ammonia poisoning is a common problem in fish tanks – especially new ones. It can cause high levels of stress in your fish and lead to other health issues including bacterial disorders.

If you see your fish gasping at the surface with red or inflamed gills, they may have ammonia poisoning. The water in their tank may appear murky, which is an indicator that the water hasn’t been ‘cycled’. To treat this condition, test the water for ammonia and pH, and check the water temperature. A 30 to 50 per cent water change is advised and you need to repeat this within 24 hours as necessary.

Clean the gravel in your fish tank with a gravel siphon and make sure you’re using high-quality food that’s not out of date. To prevent this condition avoid overfeeding, overcrowding and ensure your filtration system is working well.

Cotton mouth

This bacterial infection can affect cold-water and tropical fish. It’s sometimes confused with a fungal infection because of the appearance of white or greyish white spots on the fish’s head, though usually this infection starts as a pale area around the head and mouth. This may turn yellow/brownish in colour with red-tinged edges.

Bacteria generally affects the fish’s mouth, but lesions can appear on the back that look like a saddle on the fish’s body. Some ways to treat this condition include a 30 per cent to 50 per cent water change with a siphon of the tank’s gravel. Aquarium salt can be used, but ensure that your fish can tolerate it. You can also use Furan 2, Melafix or a vet-prescribed antibiotic.

In addition, you should remove the carbon from your tank’s filter during treatment. This infection is one of the reasons you need to use a quarantine tank when introducing new fish to your established aquarium.


Usually fatal to fish, Dropsy is characterised by a swelling of your fish’s abdomen, sometimes causing their scales to stick out. Your fish will appear listless and lose their appetite. If you’re unsure if your fish has Dropsy, quarantine them them immediately and take them to your local Greencross Vets.

Maintaining your fish tank’s water quality is key to keeping your fish safe from this condition. Also, be sure to feed your fish high-quality food and avoid overcrowding them. Test the water frequently and observe your fish for signs of stress.

Fin/tail rot

If you see that your fish’s fins or tail are frayed and the edges appear white, then they may be rotting. Your fish may also settle at the bottom of the tank and stop eating. This can be caused by poor water quality, but it can start with bullying from other fish. If your fish is being bullied, you should move them to the quarantine tank.

You can treat this condition by checking the tank’s water condition and doing a 25 per cent change of the water, along with a gravel siphon of the bottom of the tank. You can treat fin and tail rot with Furan 2 and Melafix or with an antibiotic that your Greencross Vets can prescribe.

Fungal infections

Fungal infections are one of the most common disorders for fish. Fungal spores naturally populate fish tanks, but sick, stressed or injured fish can cause a dangerous increase. These infections manifest as a white cotton-wool-like growth on the skin, mouth, fins or gills. This condition is normally a secondary problem, so it will need a two-part treatment.

You’ll first have to treat the underlying disease or injury via the infected fish spending time in the quarantine tank. Then go about cleaning your tank. A 30 to 50 per cent water change and a siphon of the tank’s gravel is necessary. Aquarium salt can be used, provided your fish can tolerate it and an anti-fungal agent will serve you well.

Be sure you always treat your tank hygienically by regularly changing your nets. Always wash and rinse your hands prior to and after handling the tank’s elements.

Hole in the head

Cichlids, and in particular Discus and Oscars are the fish breeds most commonly affected by this disorder. Vitamin deficiencies, poor-quality food and unhealthy water conditions can contribute to this ailment. You’ll notice lesions that appear as small pits in your fish’s head and on the lateral line. If you notice such lesions, perform a 30 to 50 per cent water change, checking the pH and water temperature, too.

Then improve your pet’s diet with a high-quality flake food and a vitamin supplement. Add AquaSafe Water Plus or StressCoat to improve your pet’s slime coating and help them heal. If they don’t improve, talk to your local Greencross Vets.

Ich or white spot

This is a nasty parasite that can be fatal if left untreated. At first you’ll notice white spots that look like your fish has been sprinkled with salt. You may notice your fish rubbing against objects in the tank in irritation. They may also rapidly move their gills, be lethargic or sit at the bottom of the tank. You’ll need to treat the whole fish tank to get rid of this parasite, which has a free-swimming stage to its life cycle. Follow these steps for tropical tanks:

  • Slowly increase the temperature of your tank over 24 hours to 29°C to speed up the life cycle of the parasite
  • Keep the tank at this temperature while treatment continues
  • Treat with a specific White Spot cure and follow the instructions on the label
  • Remove carbon from your filter for the duration of the treatment period as carbon will absorb any medication you add to the water
  • Treat for the recommended amount of time even if you feel that it has passed, as there may still be parasites floating in the water

Cold-water fish will need to be treated with medication and aquarium salt as the temperature increase method will cause them stress. Be strict with quarantining new fish to avoid this nasty parasite.


Though it’s not considered a disease, Popeye is more a symptom of an underlying problem. True to its name, Popeye causes a build up of fluids either behind the eye or in the eye itself. This condition can be caused by fighting, a bacterial infection, or poor-quality water.

If you notice an issue with your fish’s eye, be sure to remove any sharp objects from the tank and observe them to see if there is an aggressor. It’s best to keep your fish in the quarantine tank until they’ve healed and feed them high-quality food. Be aware that there may be an underlying bacterial infection, so don’t skip the quarantining step. You may have to treat the entire tank. We recommend a 30 to 50 per cent water change and treatment with an antibacterial option, such as Furan 2.

Slime disease

This parasitic disease is characterised by an overproduction of mucus coating, which appears as a grey/white to blue mucus coating. You’ll also see rapid breathing if your fish has this ailment, which is caused by fish feeling stressed.

Bear in mind that if the mucus covers the fish’s gills, they can suffocate. Stress can be increased by poor water conditions, overcrowding or sudden changes in temperature. If you notice the symptoms, you should check your water for pH levels and perform a 30 to 50 per cent water change. Definitely remove your activated carbon before adding any treatments, such as Furan 2 or Melafix. Use aquarium salt with caution. Preventing this disease relies on you maintaining healthy water, avoiding overcrowding and checking your fish for signs of stress.

Swim bladder disorder

The swim bladder of a fish helps to maintain their buoyancy. Issues in this region are often down to such things as constipation and air gulping. This condition mainly affects fish such as the fancy breeds with globoid bodies – Orandas and Fantails. You may see symptoms such as swimming erratically, difficulty swimming to the bottom of the tank and floating to the surface.

In some fish, this issue is caused by an underlying bacterial or fungal infection, but it can also be attributed to a kidney or liver disorder. If you notice the symptoms, you should check the chemistry of your water and perform a 25 per cent water change.

Don’t feed your fish for up to 48 hours and once that time frame has elapsed, feed them thawed frozen peas (skinned), which will act as a natural laxative. If there is no improvement, treat the tank with aquarium salt and Furan 2. If your fish sit at the bottom, reduce the water level to about half to lessen the pressure on them.

If your fish are floating at the surface, reduce the filter current so they aren’t battling against the elements. To prevent this condition, we advise soaking your pelleted food so your fish don’t gulp air at the surface and to feed them a mix of frozen and dry food.


Ulcers most commonly affect cold-water fish, such as goldfish. The ulcers are an inflammation of the external tissues that look like sores. They can be caused by physical injury, parasites, bacterial erosion, bacterial sepsis, bacterial infection or chemicals contained in poor water chemistry, such as high ammonia, nitrate and high or low pH levels.

Be sure to work out if it is one fish or multiple fish that are affected, as the latter will indicate an environmental problem. Seeing the symptoms should lead you to check your water conditions and perform a 30 to 50 per cent water change.

Quarantine the affected fish and treat with Melafix, following the dosage instructions on the label, and use different nets to avoid contamination. You might want to add aquarium salt – if your fish can tolerate it – but keep in mind that a visit to your local Greencross Vets might be on the cards.


Seeing spots on your fish, in finer yellow, rust or gold dusty tones may point to a Velvet diagnosis. It can be difficult to see, but using a flashlight on your fish in a darkened room will help you discover the parasite on your fish’s fins and gills. Watch out for rapid gill movement and signs that your fish is flicking against the surfaces of the tank.

Once the disease has progressed, the fish will become lethargic, lose weight and display laboured breathing. To rid your tropical tank of the disease, slowly raise the water temperature to 29°C over 24 hours and turn off the lights while treating. If you have a cold-water tank you should use medication as increasing the temperature will stress them. We advise you do a 30 to 50 per cent water change and add aquarium salt, provided your fish can tolerate it.

Also treat with Acriflavine and avoid carbon filtration during this period. Prevention is definitely the key with this disease, so be sure to quarantine new fish and maintain high standards of water health. Be sure to talk to the team at your local Petbarn store about the best ways to avoid this disease.