Tail and fin rot is is an easily recognizable, and very common fish disease.  It occurs in aquariums, ponds, fish farms and the wild - both in rivers and in oceans.

It is a serious fish disease that can eat into the body of the fish after destroying all of a fin, leading to death.  Even before it gets to that stage, it can lead to severe complications.  Very often, a fungal infection takes hold in the affected fin, making treatment even more problematic.

Symptoms

The first signs of tail and fin rot are red streaks in the fins.  The edge of the fin will then lighten, before it starts to rot away and gain a ragged appearance.  Small bleeds may also be present.

Biological symptoms include variations in the levels of calcium and haemoglobin in the fish’s blood.

If a fungal infection occurs, the edge of the fin may be smoother i.e. get eaten away in a more uniform manner - and have a white edge.

Causes

Tail and fin rot is caused by bacteria.  The specific bacteria that cause the disease are common and found in most bodies of water, including ponds and aquariums.  The problem arises when the fish are subjected to an external event or environment that make them susceptible to infection.

By far the most common factors are poor water quality (especially raised levels of ammonia and nitrite), and direct trauma to the tail and/or fins.  Direct trauma can be a result of nipping by other fish, bad handling by humans, or sharp edges on plants and ornaments in the water.  The stress of any of these events leaves the fish open to infection.

There are three different bacteria that can cause tail and fin rot:

1. Aeromonas - a rod shaped bacterium that can also infect humans, often resulting in the need for amputation.
2. Pseudomonas - another rod shaped bacterium that has “superbug” strains found in hospitals that cause necrotizing inflammation in humans.
3. Flexibacter - a bacteria that normally only affects fish, and is best known for strains that are bright yellow to the naked eye when colonies grow large enough.

To be precise, tail and fin rot is not a disease itself.  It is actually just a symptom of infection with one of these bacteria.

What the bacteria do is congregate in the intestine of the fish.  They then produce a mucous that contains molecules that are toxic to fish.  These toxic substances are absorbed through the wall of the intestine in much the same manner as essential nutrients, thereby entering the bloodstream.

The red streaks that are the first sign of tail and fin rot are caused by small blood vessels in the fins breaking apart under the action of the poison, and releasing blood.

Without a blood supply, the fins quickly lose their colour, break down and rot away.

While all this is happening on the outside of the fish, similar processes are destroying the blood vessels of the stomach, liver and kidney.

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