Fish diseases are always problematic subject for any aquarium hobbyist. Unfortunately, it can become an epidemic and lead to dead of all stock causing huge lost and disappointment in a hobby that we all dedicate so much time and care.

There are many articles and books about fish diseases and different treatment methods but to make a correct diagnosis and also on time – isn’t easy task even for experienced fish keeper. In most cases only microbiological and microscopic methods of diagnosis can confirm correctly what exactly pathogen (s) we have to deal with. In first stages most of diseases could show no obvious visual symptoms and in later stages disease can complicate and harm fish not giving us enough time to choose the correct medication. Incorrect diagnosis sometimes could be fatal to fish as well as incorrect treatment.

In human medicine a lot of dangerous epidemic diseases became much better controlled after very strict and organized measures were taken to prevent them. Disease prevention plays huge role in a human’s and animal’s life. It’s the smartest action that could be done to achieve the best results in aquarium hobby and trade. Prevention is always better than cure!

Main reasons that cause fish diseases are:




Let’s have a look at each of these reasons a little bit deeper.


Stress could be primary reason for most of fish diseases and it’s is very closely related with environment. Main factors causing stress are listed below.

Poor water quality. It’s very important to test source water (tap, rain, well, etc.) used for aquarium. Tap water can contain heavy metals, disinfectants, rain water could be largely polluted from atmosphere, contain toxins. Surface water (sea, lakes) could be contaminated with parasites and pathogens, pesticides and other chemicals. Well water may have low oxygen, dangerous sulphides, etc.

It is recommended to test source water at least once in 3-4 weeks. And make sure that test kits or other tasting equipment are giving accurate readings (few different test kits could be used to compare).

Water could be filtered, treated with chemicals or just simply left for at least 24 hours to aerate before used. In – tank water must be checked regularly as well.

Dissolved oxygen, temperature, salinity, pH, hardness (GH), alkalinity (KH), ammonia (NH3, NH4+), nitrite (NO2--), Nitrate (NO3-), hydrogen sulfide (H2S), should be monitored every week. Good sanitation is also very important to keep good water quality.

Allot of fish diseases can be caused by incorrect temperature, hardness, wrong food, high ammonia, dissolved organics, low oxygen, and lack of trace elements. All these factors can be avoided maintaining good water quality.

Many aquarium organisms such as bacteria and fungi usually live on the bottom of the tank and multiply. Regular water changes, bottom vacuum, regular cleaning of decorations, glass, filters, pipes, etc. reduces diseases risk. UV light could be used to reduce pathogenic microorganisms in the water.

Self-cleaning biological circle could be very effective achieving disease free aquarium. Efficient biological filtering speeds up the breakdown of waste products, eliminates excess of poisonous organic products (breaks them to less dangerous ones that could be eliminated as gasses).

Environment plays very important role in fish disease prevention. There are many factors such as fish incompatibility, tank size and position, improper filtration, too little oxygen, overfeeding, over crowding that can cause environmental disorders and diseases.

Nutrition. Poor nutrition weakens fish immune system, resistance to pathogens and also leads to nutritional disorders such as stunted growth, deformities, fading of colour, fatty degeneration of the liver and other. Various and good quality foods should be used, enriched with vitamins and vital elements. Live foods could cause parasitic and bacterial infections. All live foods should be washed carefully and microscopic examination should be carried out regularly as they often are the main source of parasites and pathogens in the aquarium. Nutritional value of live food such as artemia for example not always is the same in different stages of their life. Home made foods such as beef heart should be mixed with supplemental vegetables and vitamins. It should be enough calcium, phosphorus, vit. D, and B as well as proteins in fish diet.

Other fish stress factors such as drastic temperature, pH, hardness change, not enough hiding places, too much direct sunlight, and too much activity around the tank can also affect fish developing constant stress and weakening natural fish defences.


There are other fish disease factors (non pathogenic and non stress related) such as pathologic changes in a fish body – tumours, growths, lymphomas, cysts, double organs, misshaped body parts, skeletal deformities etc. It could be transmitted genetically, could be result of improper breeding, dying fish. It could also be caused by a certain chemicals, hormones, toxins etc. Best prevention in this case - examine the fish well before buying, always avoiding dyed fish.

PATHOGEN (or infections agent) that causes disease or illness to its host. (From Wikepedia).

It’s amazing how many bacteria, viruses, fungus and parasites could be found in aquarium. And it’s not an open system! How do they appear in aquarium causing so many problems? What can we do and what should we do to prevent pathogens and to protect our fish?

  1. Buying vaccinated fish

Mostly this applies to Koi fish and it’s supported and controlled by the government of the country to prevent large infections that can spread into natural environment. In ornamental fish this preventative matter isn’t widely available yet.

  1. Fish selection

Selecting healthy fish is the first right step in preventing diseases. Especially if fish is planned to be used for breeding. Only healthy fish should be selected. How do we know is the particular fish that we buy from local shop or breeder healthy? Before examining the fish we should pay attention to all fish tanks (are they clean, not overloaded, heated, enough light, etc.). If there is any dead fish in tanks it should be good enough reason to find better supplier. If we are fully satisfied with the whole setup, let’s try to examine the fish we are planning to buy.

Behaviour - healthy fish shouldn’t be hiding, swimming at the surface of the water, making irregular movements, lying on the bottom (except some particular species like cat fish), scratching to the subjects.

Deformities – generally any deformities – mouth, forehead (except some species), eyes, gills, fins, irregular scales etc. should not be tolerated. It could be genetic trait or developmental fault at any stage. In any case, it’s not worth to take a risk as it could be sign of a chronic disorder and if it’s genetic it might be given to future generations.

Colour – darkened colour indicates stress. It could be also sign of parasitic infection. Sometimes fish can darken during spawning so it’s important to observe and to understand what can cause darkening of fish body.

Pale colouration could be sign of a chronic bacterial infection, dehydration. Too bright, unnatural colour could mean that fish has been dyed. Never buy dyed fish as all immune system of such fish could be affected and problems might occur much sooner than you expect!

Fin and tail damage – damaged fins and (or) tail could be caused by bacteria, improper water conditions, parasites, fish fighting or several of these causes.

Pale or white patches on skin – it’s important not to miss this one as some fish keepers don’t pay attention to these symptoms especially if they are not very strongly expressed. It’s a sign of infections, parasites, fungus.

Turbidity of skin – excess slime, milky, irritated skin could be a result or poor water quality, parasites, bacterial infection, mechanical injuries.

Red patches – if red patches are noticed on fish skin – bacterial infections are highly expected.

Holes, craters – holes could be obvious sign of a very poor water, improper diet, bad selection and breeding. In some cases it could be bacterial infections, parasites, fish aggression.

Spots, dots – when selecting fish any salt-like dusting should not be accepted. This could be an indication of white spot disease or viral infections.

Fungus (cloth –like growth) – it’s definitely not worth to take home!

Eyes – should be normal size, not cloudy or popped out. Bulging, deformed, protruding eyes are also symptoms of stress or disease.

Swollen body – usually it’s accompanied with other symptoms (unusual behaviour, darkened body, lethargic movements, lying on the bottom, etc). In most cases it’s a result of bacterial infection. Don’t ever take a risk to buy such fish!

Obvious parasites – lice or (and) worms.

  1. Quarantine

Fish should be ALWAYS quarantined before introduced to the tank. Quarantine should last 3 to 8 weeks depending on quality, origin and species of fish. Best results could be achieved in a separate bare tank supplied with heater, filter and oxygen. Decorations or gravel are not recommended. It’s better to keep quarantine tank in a different room from the main one and précised hygiene is required for all instruments and for the caretaker itself. Antihelmintic treatment in the main tank should be carried out 2-3 weeks before new fish arrive. After treatment is done and large amount of water has been changed, quarantine tank should be filled with the water from the main tank not earlier than few days before new arrivals. Make sure that quarantine tank was disinfected after it has been used last time including all nets, filters, pipes, etc. Potassium permanganate powder could be used for this purpose. (More detailed instructions of use would be on the manufacturer’s package). Some people might say that quarantine isn’t necessary if fish comes from good place and looks perfectly healthy. Let me give you one example. Let’s say you had a white spot infection in your main tank a good while ago. Your fish, after successful treatment survived infection and now have immunity for it. The parasites formed a latent stage at protected hiding places such as gills or fin bases. Newly introduced fish can suddenly become infected and you’ll believe that infection was brought with the new fish. But in fact, new fish was infected by the latent stages of the parasites that were already in the tank! In this case treatment will be more difficult because fish are already stressed in new environment and didn’t acclimate yet. Also, treatment in main aquarium would be complicated because of huge amount of biomass, plant, filters, etc.

  1. Prophylactic - quarantine treatment

During the quarantine general treatment including anti worming could be carried out. Antibacterial treatment could be considered if presents of parasites are obvious, as very often it causes secondary bacterial diseases. If fish are in poor condition, symptoms of possible bacterial infection are confirmed, antibacterial treatment should be started immediately. If it’s possible to get somebody to take microscopic samples for parasitic infection diagnosis, it should be done before any treatment is chosen and started. Antibacterial treatment should be carried out for at least 7 days followed by a large water change and after that antihelmintic treatment should be carried on. Very important not to mix different treatment together because they can affect each other’s effectiveness and form very toxic chemical substances that can actually kill fish faster than disease itself. Medication for antibacterial treatment should be recommended by professional but if this is not possible, more experienced adviser can help. Dosage should be strictly as it says on the package and measured precisely! A lot of human antibacterial medicines could be used for fish, but not in case if symptoms are unclear and no microbiological or microscopic diagnosis were confirmed.

Parasitic treatment could be subdivided into external and internal parasites treatments.

External parasites (non encapsulated) could be treated with salt, formalin, copper sulphate baths. External multicellular parasites (skin, gill flukes) could be treated with Febendazole, Flubendazole (Flubenol, Wormer Plus), Mebendazole, Praziquantel.

Internal free swimming parasites (Hexamita) could be treated with Metronidazole. Encapsulated unicellular internal parasites (Ich, velvet disease) could be treated with copper sulphate, malachite green, formalin.

Internal worm infections could be treated with Levamisole, Febendazole, Piperazin, Praziquantel, garlic (by mixing in a food).

If you are not sure what parasites your fish might carry, better apply combined – staged anti worm treatment, example Wormer Plus and Praziquantel. (Not at the same time). It’s very important to follow dosage recommendations and to do large water changes after each treatment. Please note – Praziquantel can not be used together with formalin.

If not treated properly during the quarantine fish can contaminate each other later even if they don’t show any symptoms of disease.

There are many methods of fish quarantine and it depends on origin and species of fish. If it’s carried right it saves fish life as well as our money and time. Prevention is always better than cure!


- Gerald Bassleer (2003): The new illustrated guide to fish diseases, Belgium

- Dieter Untergasser (1989): Handbook of fish diseases, USA

- Treves Brown (2000): Applied fish pharmacology, The Netherlands

- Andrew Soh(2005): Discus, the naked truth, Singapore

- Shubert ((1978): Cure and recognize aquarium fish diseases, USA

- Roberto Hansen (2006): Water quality in the ornamental aquatic industry, The Nederlands