The aquarist must learn how to detect signs of ill health, and above all establish good habits as regards tank maintenance and food hygiene, in order to prevent disease. A fish can not only fall sick, but can also be a carrier of a disease without actually being sick itself- then it is known us a healthy carrier. The disease will appear under certain conditions or will be passed on to its companions.


The origin of a disease can be outside the aquarium

- the introduction of a fish stressed by importation, or by the living conditions at a retail store; it can fall sick and contaminate other fish;

- the introduction of a healthy carrier or diseased fish;

- the introduction of polluted water from a natural source;

- the accidental introduction of various harmful substances, such as cigarette smoke and aerosol fumes.

Obviously, every precaution must be taken to avoid such accidents on the part of the aquarist.

The origin of a disease can be inside the aquarium

This is generally due to a disruption of the general balance, leading to the development of the diseases latent in a healthy carrier:

- reduction in temperature or thermal shocks, i.e. abrupt variations - whether increases or decreases - brought about by a disorder in the heating system;

- reduction in the oxygen levels;

- excess of nitrogenous matter, due to a filter malfunction or too many fish;

A small tank of glued glass is used, containing only the equipment necessary for treatment. Therefore, there are no plants, bed, or decor, except in the case of naturally shy fish, which can be furnished with a shelter made of an artificial material like PVC. Normal filtration is not required; the most that is needed is a small internal filter containing only Perlon cotton. On the other hand, there must be substantial aeration, as this influences the oxygen levels. The temperature should be raised to 27-28°C.

Ideally, the hospital aquarium should be in a quiet spot with little light, in order to enhance the healing process. It is also possible to cover the glass sides.

After it has been used, both the aquarium and its contents must be disinfected, using 4 ml of bleach for every 100 liters of water, followed by stirring of the water over a period of 24 hours and then several successive rinses.

• Scatophagus argus, in a poor condition, with damagedfins.

- the general quality of the water;

- an injury sustained in the course of a fight or an overexcited mating ritual, or from collisions with or scrapes against the decor;

- underfeeding or a poorly balanced diet.


Prevention is better than cure: this wise old proverb is perfectly applicable to fish-keeping. Prevention entails daily observation of your fish and their environment, and this requires a certain degree of knowledge. In the end, the best prevention comes from maintaining a good balance in the aquarium.

Diseases and their treatment

Generally speaking, there are two types of diseases - infectious and non-infectious. The former are caused by microorganisms, such as bacteria, fungi, or viruses carried by the fish. The triggering factors are well-known: stress, bad diet, decrease in temperature. Some pathogenic organisms have a mixed life cycle: one part on the fish, one part in the water. Non-infectious diseases are not caused by pathogenic organisms, but by the environment (low-quality water, underfeeding). It is important to avoid any cocktails of medicines and useless or harmful overdoses. The treatment schedule must always be respected, even if the symptoms quickly disappear. When taking care of sick fish, it is best to feed them moderately, but with natural foodstuffs. Once the illness is cured, provide a varied and well-balanced diet, in order to consolidate the healing process.



r ' 1 Diseases are not particularly easy to detect, especially if they do not manifest any external symptoms. However, several indications can lead an aquarist to think that a fish is sick. These phenomena can be seen in isolation or together.

- General behavior: the fish hides, is easily scared, and is stressed out.

- Swimming: uncoordinated, and the fish may scrape against the decor or the bed.

- Feeding: weight loss, refusal of food.

- Breathing: the fish comes to the surface to "stock up" on air.

- External symptoms: white spots, whitish marks, swelling of one or both eyes or the whole body, bristling scales, several wounds, etc.

The bearing of this fish (Macropodus opercularis) and its siightiy raised scales suggest a poor state of health.

There are two things to take into consideration: the fish's appearance and its behavior. This requires a good knowledge of its anatomy, biology, and ecology. A fish in good health has bright colors and sparkling eyes. Its body is not swollen and its scales and opercula do not stick out. It comes to eat in a normal way, in terms of both the feeding process itself and the amount it eats. It does not hide without a good reason.

Bright colors and fully deployed fins are signs of good health.

Bright colors and fully deployed fins are signs of good health.

The most common diseases and their treatments are listed in the table on pages 70-71.

The stages of treatment

1. Put the fish in a hospital aquarium.

2. Raise the temperature to 27-28 °C.

3. Stop the filtration, and maybe the protein skimmer in salt water, while at the same time increasing the aeration.

4. Dilute the medicine as required.

5. Pour the medicine little by little into the aquarium. It is best to spread this operation out over a period of at least one hour, or one day when administering copper sulfate.

6. Leave it to act for the recommended period.

7. Empty out half the aquarium, then top up by adding water that is identical to the original.

8. Change 10% of the volume each day for 5 days, always using an identical water.

9. Repeat the treatment if necessary, following the steps above.

10. Switch the filter on again and, where applicable, the protein skimmer.

11. Gradually reduce the temperature to its original level. This operation must be spread over 3 days.

12. Bring the aeration to its original level. In a mixed aquarium, the process obviously begins at point 2.


Know your aquarium; regularly analyze certain parameters - nitrites, pH; have good-quality water;

- Know your fish and their habits - behavior, feeding.

- Avoid overpopulation.

- Avoid both overfeeding and underfeeding;

- Avoid any permanent stress, particularly from mixing fish that are incompatible due to their size or behavior, or from a lack of shelters and hiding places.

- Do not introduce new fish directly into the aquarium.

• Powders such as copper sulfate and methylene blue are weighed and then diluted in water to form a mother solution.

Knowing your fish well - their color and behavior -constitutes one of the main rules for disease prevention in an aquarium. •

Knowing your fish well - their color and behavior -constitutes one of the main rules for disease prevention in an aquarium. •

• Powders such as copper sulfate and methylene blue are weighed and then diluted in water to form a mother solution.


There are a great many medicines on the market, specifically formulated for one or several diseases. Constant progress is being made in this field, with new medication for marine fish also being produced in the last few years. It is important to respect the instructions regarding both the dosage and the treatment schedule. Bacterial diseases can be combated with antibiotics. However, the use of these substances, which are difficult to obtain, is not recommended, and could lead to the creation of resistant strains. Various chemical products can be used in treatment, provided the dosages are fully adhered to.

Firstly, and only to be used in fresh water, there is kitchen salt, which is effective in certain cases. It must be added gradually to the water until a level of 5-10 g/liter is reached, but this dose must not be exceeded. Once the fish is cured, the water is returned to its initial unsalted state, by means of successive changes of 25% of the volume of the tank per day. Methylene blue is efficacious against fungi. Dissolve 1 g of powder - which can still be found in some pharmacies - into 1 liter of water. Treat fish with 0.5-1 ml/liter of this solution, and eggs with 1 ml/liter. The product is then eliminated through gradual changes of 25% of the volume of water. Methylene blue cannot be used in sea water. Malachite green is particularly effective against fungi, but also against ichthyophthyriasis (white spot disease). A solution of 1.5 g per 10 liters is prepared just before


A The quantity required for treatment is taken from the bottle on the left, which can then be stored.

use. and the treatment should not last more than 2 hours.

Formol is active against external parasites. A commercially available 40% solution of formaldehyde is used, after diluting at the rate of 20 ml/100 liters of water. Generally speaking, 15-20 minutes of treatment is sufficient, but in any case it must never exceed 30 minutes. The treatment can be repeated twice, at 48 hour intervals. (Coppersulfate is often used in sea water, but it is toxic for invertebrates and some plants. The ailing fishes must therefore be treated in a hospital aquarium if they are not the sole occupants of the marine tank. Dissolve 16 g of the crystals into 1 liter of water; the treatment dose is 10 ml of this dilution per 100 liters of water. As a precaution, the product's introduction into the water can be spread out over one day. The dilutions of methylene blue, malachite green, and copper sulfate must be carried out with distilled water. All these products, apart from the salt, must be kept in a cool, dark place - ideally, in the refrigerator.

However, be aware of any possible dangers, especially to children, who might be attracted by the colored liquids formed by copper sulfate and methylene blue.


There are very few aquarists who have never been faced with diseases associated with white spots or fungi.


In the former case, the infection is caused by a protozoon (unicellular animal) which alternates its existence between the fish and the water. It is extremely contagious. This disease is particularly likely to develop when there is a drop in temperature, or after the introduction of a new fish. It is treated with commercially available products or with formol. The same conditions also apply to the "foam" caused by fungi, including the Saprolegnia. Its proliferation is enhanced by skin wounds. The treatment is carried out with commercial products, or with either methylene blue or malachite green.


This is caused by a unicellular parasite that bears a flagellate, which allows it to move about. It alternates its existence between the water and the fish. This very contagious disease appears when a fish has been weakened, as up to then it was a healthy carrier. It can be treated with copper sulfate or with a commercial medication.



Name of disease or problem


Type of water

White spots on the body and the fins (the size of a pinhead, 1 mm maximum).


Protozoon parasite, Ichthyophthirius.

Fresh water.

White spots, smaller than the above, forming a fine veil, the fins often stuck, scraping against the decor.


Unicellular parasite, Oodinium, equipped with a flagellate to move around.

Fresh water, but above all in sea water.

Identical to the above, but with bigger spots.


Protozoon parasite, Cryptocarion.

Sea water.

White clumps with a cotton-like appearance, foam.



Fungi, including Saprolegnia.

Fresh water, rare in sea water.

1 or 2 eyes abnormally swollen.


Bacteria, viruses, fungi, sometimes all together.

Fresh water and sea water.

Swollen abdomen, bristly fins.


Mainly bacteria.

Fresh water and sea water.

Opercula sticking out, unsteady swimming, tiny flukes on the branchiae.


Parasitical fluke, Gyrodactylus.

Fresh water.

Gasping for air at the surface.

Lack of oxygen.

Defective aeration, general balance of the aquarium disturbed.

Fresh water and sea water.

Weight loss, poor growth.

Dietary problems.

Underfeeding, or lack of vitamins.

Fresh water and sea water.

Agitation, uncoordinated swimming, unusual behavior.

Bad maintenance conditions.

Oxygen problems, poor quality of water (especially nitrogenous substances).

Fresh water and sea water.

Fry with deformed skeletons.

Hereditary problem.

Genetic origin (the parents).

Fresh water and sea water.

This Neolamprologus multifasciatus has foam on its wounds

This Neolamprologus multifasciatus has foam on its wounds




Increase in temperature, formol, malachite green, commercial medication.

Contagious, appears if there is any significant drop in temperature.

Copper sulfate, commercial medication.

Sometimes difficult to detect at first, common in sea water, contagious.

Coppersulfate, commercial medication.

It is sometimes associated with small bloody spots; fish can experience breathing difficulties.

Salt, malachite green or methylene blue (except in sea water: copper sulfate, commercial medication).

The disease's development is favored by wounds.

Coppersulfate, commercial medication.

Sometimes difficult to treat.

Coppersulfate, commercial medication.

Contagious, sometimes difficult to treat.

Salt, copper sulfate.

Not very easy to detect.

Increasetheaeration,checkand adjusttheparametersofthe water.

Gasping for air can also be a symptom of an infectious disease.

Fresh food alternated with live prey.

Possibility of incorporating commercial vitamin solutions into the food.

Increase thequantity of oxygen, cnange one third of the water, check its quality (nitrites, pH).

The symptoms can also correspond to an infectious disease.

If a lot of fry are affected, there is a genetic problem - the parents, and they must therefore be separated.

It is not unusual for a few of the fry in a batch to be affected.

• Fish afflicted by white spot disease (ichthyophthyriasis). Easy to detect - the body is covered with white spots - this disease is very contagious.

• A fish's swollen abdomen is often the sign of dropsy, a bacterial disease.

• Fish afflicted by white spot disease (ichthyophthyriasis). Easy to detect - the body is covered with white spots - this disease is very contagious.


- Act immediately.

- Administer the appropriate treatment.

- Take care of the fish in a hospital/quarantine aquarium. Treatment must be given as soon as the disease appears, i.e. when you see the first symptoms, and you must act quickly.

Treating a sick fish in a mixed aquarium is not without its risks: some substances can have undesirable side-effects on other species or on the plants. It is therefore preferable to use a hospital aquarium, or a quarantine aquarium.

• A fish's swollen abdomen is often the sign of dropsy, a bacterial disease.

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