Most common fish diseases: How to identify them, prevent and treat all fish sicknesses


What Is Fish Health Management?

Fish health management is a term used in aquaculture to describe management practices which are designed to prevent fish disease. Once fish get sick it can be difficult to salvage them.

Successful fish health management begins with prevention of disease rather than treatment. Prevention of fish disease is accomplished through good water quality management, nutrition, and sanitation. Without this foundation it is impossible to prevent outbreaks of opportunistic diseases. The fish is constantly bathed in potential pathogens, including bacteria, fungi, and parasites. Even use of sterilization technology (i.e., ultraviolet sterilizers, ozonation) does not eliminate all potential pathogens from the environment. Suboptimal water quality, poor nutrition, or immune system suppression generally associated with stressful conditions allow these potential pathogens to cause disease. Medications used to treat these diseases provide a means of buying time for fish and enabling them to overcome opportunistic infections, but are no substitute for proper animal husbandry.

Daily observation of fish behavior and feeding activity allows early detection of problems when they do occur so that a diagnosis can be made before the majority of the population becomes sick. If treatment is indicated, it will be most successful if it is implemented early in the course of the disease while the fish are still in good shape.


Virtually all fish diseases can be traced to stress, which weakens their immune systems. Sources of stress include shipping, poor water quality, improper water chemistry, inadequate filtration, improper diet, overcrowding, leaving the light on 24/7, injuries, aggression from other fish and inadequate habitat. To keep your fish in optimum health, perform regular partial water changes, be diligent about filter maintenance, feed them a varied high-quality diet, don’t overstock your aquarium, and put the light on a timer to simulate a normal day/night cycle. When performing water changes, always treat tap water with a conditioner before adding it to your aquarium.

Never purchase fish that have just arrived at your local aquarium store. New arrivals are usually stressed and moving them again will only stress them further. Let them settle in for a week or more before purchasing. Take new purchases straight home and acclimate them to your aquarium’s pH and temperature for at least 30 minutes, spending extra time with sensitive species or if the store’s water chemistry is significantly different from yours. Make sure there is plenty of cover for all your fish and rearrange decorations if necessary to deter established territorial fish from harassing new arrivals. After adding new fish to your aquarium, leave the light off for a few hours to allow them to adjust to their new environment. Do not tap on the glass or suddenly turn on the aquarium light in a dark room.

While most freshwater aquarium fish are raised in captivity today, some rarities and oddball fish are still collected in the wild. These fish may have a higher chance of carrying diseases and have typically gone through far more stress on their way to your aquarium. Extra care and quarantine measures should be taken when purchasing wild caught fish. When keeping burrowing fish such as eels in the genus Mastacembelus and certain types of loaches, use sand instead of gravel to avoid injury to their bodies when they burrow into the substrate. Use smooth, rounded gravel to prevent Corydoras catfish, loaches, goldfish and other species that like to forage on the bottom from injuring their barbels and mouths.

An effective way of maintaining a disease-free aquarium is to quarantine all new additions. While this may not be practical for every aquarist, it’s well worth the investment for those who keep high value fish such as discus, rare fish or dedicated planted aquariums where the use of medications is not recommended. Quarantining new fish greatly reduces the chance of introducing a disease organism into your aquarium and allows you to safely treat sick fish if necessary, without introducing chemicals to your show tank. A quarantine aquarium can also be used to isolate bullies or fish that are being picked on.

A 20-gallon aquarium works for most situations. It should be filtered, heated, and maintained just like any other aquarium, and should be fully cycled with test fish before being used for new purchases or treatment. To provide cover for fish, decorate the aquarium with plastic plants or other non-porous decorations that are easy to sterilize and/or clean. Do not use porous rocks or driftwood as they can absorb medications. Different sized PVC fittings or sections of pipe can also be used as hiding places. Do not use gravel or substrate, as this gives parasites like Ich a place to reproduce.

Remove carbon and other chemical media from the filter, as they will adsorb medications, lowering their effectiveness. An aerator is recommended in addition to the filter, as some medications lower the oxygen level inthe water. A light can be used but is not necessary, as dimmer surroundings will calm your fish and are also known to inhibit some disease-causing organisms.

Whenever you purchase new fish, place them in the quarantine aquarium for a minimum of 30 days to make sure they aren’t sick. Many hobbyists treat new purchases preventatively against parasites whether they are observed on their fish or not. Perform a 25% water change and filter with carbon for at least 48 hours before switching medications or introducing new fish to your quarantine station.

Have a separate net, siphon hose, algae scraper and other equipment for your quarantine aquarium, and never use them in your display aquarium. Doing so risks spreading diseases and defeats the purpose of the quarantine aquarium. Disinfect this equipment in bleach water and rinse well on a regular basis. After working in your quarantine aquarium, scrub your hands and lower arms with an anti-bacterial soap before working in your display tank.
Ultra-violet Sterilizers

Disease causing organisms exist in virtually every aquarium, but they will not infect fish if their numbers remain low and the fishes’ immune systems are functioning properly. Ultra-violet sterilizers kill disease-causing organisms as well as suspended algae and help keep water healthy and crystal clear. Coralife Turbo-Twist UV Sterilizers are available in three sizes and accommodate aquariums up to 500 gallons. UV sterilizer lamps should be changed every 10 months or 7,000 hours of operation to maintain peak performance.


In order to recognize problems that may arise, it’s helpful to have an understanding of what “normal” appearance and behavior are for your fish. Observe your fish regularly – feeding time is a good opportunity to do this. Look for white spots, cloudy eyes, bloody patches, a white body film or torn, ragged fins. Also, there are certain things fish never (or at least very rarely) do. For example, fish do not normally gasp at the surface. They typically do this because of poor oxygenation, high nitrite levels, parasites, or damage to their gills. Other behaviors that are cause for concern include loss of color, shimmying, rubbing against decorations or the substrate, cowering, refusing food, or dashing around the aquarium.

If your fish appear sick or exhibit abnormal behavior, consider speaking to an expert as soon as possible for assistance with diagnosis and treatment. Photographs or a short video of your fish can be extremely helpful when seeking advice, as verbal descriptions can be misinterpreted by aquarium shop personnel and other experts.

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


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.


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.


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 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.


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.


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.


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.


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.