Sick fish: what to do?

They live their whole life in an enclosed, small space, which favours the transmission of pathogens. If one of your fish is sick, it could be a contagious disease. Isolation or quarantine is then necessary to protect its congeners.

When should you isolate your fish?

Before any new arrivals are introduced, it is preferable to temporarily isolate them. It is then a quarantine strictly speaking. The objective is to prevent the introduction of diseases in your aquarium. Indeed, a seemingly healthy fish can still carry diseases that will reappear with the stress of transport. Most of the time these are parasitic diseases. By respecting an isolation period of 30 days, you can easily avoid the contamination of the whole aquarium. This period can eventually be shortened, but should never last less than 15 days.

On the other hand, if one of the fish in your aquarium is visibly ill, it may be important to isolate it. Isolation has the advantage of allowing close observation and targeted treatment of only those fish that are affected by the disease. To do this, you must transfer it for about ten days to a “hospital tank”, which must meet special requirements, at the risk of being an additional stress for the fish that are already in bad shape. In general, 10 to 15 days of isolation are necessary. In the absence of a hospital tank, it is sometimes preferable to eliminate the sick fish in order to avoid an epidemic that could lead to the loss of all the fish in the aquarium. Also remember to remove dead fish as soon as possible.

Isolation makes sense when you suspect a contagious disease, or when the necessary treatment cannot be undertaken in the community aquarium as it may disturb healthy fish and plants. In most cases these are diseases caused by parasites, and more rarely by fungi or bacteria. Only the most common ones are listed below.

Parasitic diseases :

Ichthyophthirius multifiliis: the “white spot disease”: This is a very common parasite in many cold water fish. It manifests itself by the appearance of white spots visible to the naked eye, about 1 millimeter in diameter, on the skin of the fish. This parasite develops very quickly in the aquarium. The disease is highly contagious and very virulent. Therefore, affected fish must be isolated quickly.

Dactylogyrus: This parasite of the skin and gills mainly affects cold water fish. Too high temperatures can promote its development. The fish become listless, stop eating, and produce excessive mucus. They sometimes try to rub themselves against decorative elements, as if to scratch themselves. This parasite is frequently introduced by new fish.

Fungal diseases :

Saprolégniacées: These fungi are responsible for the most frequent mycoses, especially in cold water fish. Affected fish are then gradually invaded by white to greyish cotton-like filaments. This disease is often secondary to poor feeding, injury or parasite infestation.

Bacterial diseases :

  • Fin rot“: This widespread disease is most often caused by the Flexibacter bacteria. Veiled fish, such as fighting fish, are the most susceptible. The disease manifests itself by the bleaching of the fins, which eventually fray. It occurs mainly on poorly maintained or weakened fish, but can also be introduced by newly purchased fish.

How to set up the quarantine tank?

One and the same tank can be used as a hospital tank for sick fish and as a quarantine tank for new arrivals. The rules for setting up and maintenance are the same.

Although it has obvious advantages in case of contagious disease, the installation of this tank has a cost. Indeed, it requires a specific material, which is exclusively intended for it (artificial plants, thermometer, landing net…). It cannot be used for any other use such as reproduction or maintenance of other fish. Indeed, any inert object can be used as a vector for diseases. The decoration of the tank must therefore be very basic, and everything must be easy to clean (for example, do not put any substrate, sand or gravel in the bottom of the tank). On the other hand some “hiding places” will be appreciated, like some plastic plants or upside down pots for example. It is necessary to proscribe the real plants, which in any case would not survive the possible treatments and the darkness.

In short, this tank should contain only the bare minimum, and any unnecessary elements on which pathogens could find refuge should be avoided. At the end of the isolation period, the aquarium and all equipment must be cleaned with a disinfectant such as bleach.

The respect of hygiene rules is fundamental. Remember to wash your hands before and after the maintenance of your aquariums.

The isolation tank must be sufficiently spacious and equipped with a lid. The size must of course be adapted to the species concerned. Any source of stress that could delay the healing process should be avoided. Ideally, it should be placed in a quiet place with little light. For the sick fish it should be a comfortable and restful place.

In order to allow the biological maturation of the water, one possibility is to use a filter cartridge previously used in the community aquarium, and to maintain the quarantine tank permanently, even when no fish are present. For the hospital tank, water can be taken from the community aquarium. In all cases the chemical parameters of the water (ammonia, nitrite, pH and temperature) will have to be adapted to the species. It is preferable to check them with commercially available test kits on a daily basis or, at the very least, at each water change.

If it is a quarantine for a fish you have just acquired, the temperature should be maintained within the high values tolerated by the species. This way, if your new fish “incubates” a disease, it will develop faster and you will detect it earlier. Conversely, diseases will take longer to develop in colder water, so the quarantine period should be extended.

If the fish is already sick, it is preferable to keep the same temperature as in the original aquarium, except for white spot disease which may require a slight transient rise in temperature.

For tropical fish, the use of a heater is recommended; a temperature of 22 to 25 degrees is suitable. For cold water fish, 12 to 15 degrees is generally sufficient. You can also place a porous stone to ensure sufficient oxygen.

Filtration is of the utmost importance here: the water must be of the best possible quality. Only a mechanical, foam filter should be installed. Carbon is to be avoided as it can interfere with the action of any treatment. As for biological filters, their operation may be affected by medication.

Generally speaking, water changes can be made twice a week, with 25 to 50% of the water volume changed each time. On the other hand, they should be daily if your fish suffers from white spot disease.

Distribute food in small quantities.

For fish in water bodies :
If you are keeping fish in an outdoor pond, the same equipment as above can be used, provided they are small. For larger fish, you will need to use a larger container, such as a children’s pool, covered with a net to protect your fish from predators. Avoid direct sunlight and place the quarantine container away from the water.

Quarantining a fish is not easy: whether it is to acclimatize a newcomer, or to care for a sick fish, the stress of the change of environment can be fatal. More than ever, special attention must be paid to water quality. Nevertheless, a well conducted quarantine remains the best way to protect your fish.