Aquarium Bible

Illness in Aquarium Fish Due To Infectious Organisms

Illness in Aquarium Fish Due To Infectious Organisms
As with any other living creature, the organs and tissues of a fish are all vulnerable to infection. With aquarium fishes however, the diseases that fishkeepers more commonly come across are those of the skin and gills, this is because these are more readily seen and diagnosed.

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Infections of the skin and gills

There are a wide variety of infections that are visible on the skin of a fish; however, the two that are most common in the aquarium are “White-spot” and “Velvet disease”. Single-celled animals or organisms known as protozoa, are the cause of both of these diseases, protozoa is a group that comprises such single-celled organisms as:

> Amoebas; a single-celled animal that catches food and moves about by extending finger-like projections of protoplasm.

> Flagellates; protozoans that have one or more thread-like structures, known as flagella these are slender thread-like structures, in this case a microscopic whip-like appendage, which enables many protozoans and bacteria to swim).

> Dinoflagellate: a single-celled aquatic organism with two flagella.

> Ciliates; a single-celled animal distinguished by a short microscopic hair-like vibrating structure known as a cilia, these occur in large numbers on the surface certain cells.

> Sporozoans; a group of mainly parasitic protozoans with complex life cycles, including the organisms responsible for malaria, toxoplasmosis, etc.

Other diseases of the skin and gills can be caused by:

> Bacteria

> Virus

> Fungi

> Helminths

> Parasitic crustacea

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Protozoal diseases

White-spot disease

White-spot disease, or Ich is caused by the ciliate Ichthyophthirius multifiliis. The body of this organism is spherical in form and is large by protozoan standards, growing up to 1mm in size. The parasite spends most of its life on a host fish, and, by the use of its cilia; it rotates vigorously burrowing into the surface layer of skin where it feeds off skin cells and surface debris. The burrowing action causes local irritation and the skin eventually grows over this enclosing the parasite under the skin, thus forming a white spot.

The parasite has a three-stage life cycle, and depending on the temperature, can take up to 20 days to complete, reproduction occurs away from the host fish:

1.) The first stage is spent growing on the skin of its host as already described.

2.) After maturing on the skin, which can take a few days to a few weeks depending on the temperature, the parasite bores its way out and drops to the aquarium floor, as a jelly-like cyst. Rapidly, within the cyst, cells start to divide, and in just a few hours several hundred individual cells have been produced.

3.) The individual “daughter” cells (swarmers) have now broken out of the cyst and have to find a new host in which to start the whole process again, unless they do this within three to four days they will die.

The disease is easy to recognise, the whole of the fish’s body will be covered with noticeable white spots. In severe cases the spots may merge together to form grey patches. The skin will become slimy and the fish will close its fins and rub its body against objects in the aquarium, eventually becoming thin and listless. Under a microscope scrapings of mucus from an infected fish would reveal the constantly rotating parasites.

All species of fish are vulnerable to attack by this highly infectious disease, although in an otherwise healthy aquarium it may be nothing more than a short-lived attack with very few parasites. There could still be a danger however, some fishes fight off the disease and become immune, these “survivors” may show no signs of the disease but may be potential carriers of it.

The real threat is when the parasite is introduced into an aquarium that accommodates less than healthy fishes due to some other factor, such as poor water conditions for instance. An already weakened fish will succumb to the infection and before you know it the parasite will have taken control, covering the whole of the fish’s body, the wounds left behind by the parasite will give rise to secondary infections, all in all this could prove fatal.

If you suddenly get an outbreak of this disease in an aquarium that is otherwise perfectly healthy, it is most likely that the parasite has been introduced either on a newly acquired fish or as a cyst, on a plant for instance. To be certain of maintaining a healthy environment for your fishes it is best to quarantine all new stock, including plants (and anything else that goes into the aquarium for that matter), they should be housed for about two weeks in a quarantine tank prior to introducing them into the main aquarium; this will go a long way to ensuring that you have a disease free environment for your fishes,


Once the parasite has been identified it is best to treat it in its third stage (free-swimming stage), this is because it is in a protected-state while on the body and in the cyst. Also since the parasites life cycle can take up to 20 days to complete, the treatment should last accordingly to ensure complete removal of it.
Reliable, pre-mixed, ready-to-use medications are available nowadays; as soon as the problem is identified you should seek out the appropriate ready-to-use remedy from your local aquatic store, and follow the manufacturers directions.
See Method of Treatment

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Oodinium or “velvet” disease

The organism responsible for what is often referred to as ”velvet” or “rust” disease is Oodinium, a dinoflagellate. There are two species of importance, O. limneticum, which gives the characteristic yellow or rust colour on the skin of the affected fish, and O. pillularis, which appears as patches of velvety grey or blue-grey colour on the sides of the fish.

Symptoms may resemble “white-spot”, although in this case Oodinium is smaller than Ichthyophthirius, the parasite being in a motile state at about 13µ (1µ = 0.001mm) - [µ = micron] and pear-shaped. At this stage it is able to attach itself to a host fish with its longer flagella, which are situated at its “pointed” end. The parasite then produces several temporary protrusions (pseudopodia), which are able to penetrate the outer layer of the host’s skin and gills off which it feeds. It is interesting to note that in free swimming and parasitic form the organism is able to obtain food by the process of photosynthesis, this is made possible due to the fact that the organism contains some chlorophyll (the green pigment of plants), and is classed as an alga by some authorities.

Once on the host fish, the parasite matures in a few days, and just like Ichthyophthirius, multiplication takes place within a cyst. It is here that the two species differ.

With Oodinium limneticum the organism rounds itself off and a series of divisions take place within the cyst, which may give rise to more that 200 young parasites (dinospores or daughter cells) in just a few days.

With Oodinium pillularis however, the parasite falls from its host and sinks to the bottom before adopting the cyst stage. In this species only 32 or 64 dinospores will be produced, each possessing two hair-like developments known as flagella, which are used for propulsion. These flagella will disappear once the parasite has found its host and produced pseudopodia with which to attach itself. The dinospores must then find a host within 24 hours otherwise they will perish.

Affected fishes have a dusty appearance on the skin and this is usually sufficient evidence of an attack. The burrowing action of the parasite causes skin damage; this could lead to heavy infection where the gills and mouth may be affected. In tropical species the operculum may swell or bleed. Inflammation, and degeneration can occur. Fishes can be seen rubbing against objects in the aquarium as they attempt to rid themselves of the irritation. They will rise to the surface and gasp for air. Severe infections often prove fatal.


The best way to prevent an outbreak of velvet disease, or any other disease for that matter, is to quarantine new stock for about two weeks before introducing them into the main aquarium.

If you are unfortunate enough to be confronted with an outbreak of velvet disease, you may encounter some difficulty in eradicating the organism, information seems to suggest that it can survive in a non-parasitic state for some time, this is because of its ability to produce its own nourishment through photosynthesis.

Treatment therefore should be carried out in total darkness, not the slightest ray of light should be visible from inside the aquarium; something like a heavy black sheet should be used to cover the tank. There is a slight chance that this alone could eradicate the parasite, with the lack of light it would probably starve to death before any harm came of the fish.

However, suitable remedies are available from aquatic stores, and once the organism has been identified no time should be wasted in treating it. This is a highly contagious disease and in the end is a killer. Species of small fish and the very young are particularly at risk.

Be sure to use a separate “hospital” tank for the affected fish, this should be void of plants and gravel, although a small air operated corner filter can be used, this will also provide aeration. It is usually recommended that the temperature be raised to 30ºC (86ºF).

Reliable, pre-mixed, ready-to-use medications are available nowadays; as soon as the problem is identified you should seek out the appropriate ready-to-use remedy from your local aquatic store, and follow the manufacturers directions.
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