Fin & Tail Rot

Fin or tail rot is a common problem in aquarium fish. It describes a variety of infections including lesions, erosion, splitting of fin rays, nodular thickening and extensive loss of tissue. It can be caused by inappropriate aquarium conditions which may increase susceptibility to infections. Often it is the result of aggressive interactions that cause fin damage and secondary infection from a range of bacteria or fungal spores. Fish having mild cases can completely regrow the soft fin ray tissue if infection is resolved.

The most common bacteria involved are usually Aeromonas, Pseudomonas and Flavobacterium. These bacteria are often natural inhabitants in the skin mucus of fish. However, definitive diagnosis is by fish necropsy, culture and isolation of bacteria. Affected fish become anorexic, lethargic and dark in colour. They generally have a rather gross appearance with frayed fin edges and reddened caudal fin in the early stages. The tip of the fin becomes greyish, and then it becomes eroded and hemorrhagic. The lesions progress into fin rot or extensive fin loss, and in more advanced cases, the caudal fin will erode back to the peduncle, with ulcerated tissue often exposing the skeletal structure. Eventually, even the muscle fibres will be affected. Secondary fungal infection is common. A high mortality rate may be observed within a few days unless rapid identification of the disease-causing pathogen and treatment of the infected fish is undertaken immediately. Mortality increases significantly once bacteria enter the circulatory system.

A research project in 2004 investigating 170 naturally infected fishes with finrot revealed clinically progressive erosion, congestion and haemorrhages of the body fins especially the caudal and dorsal fins and oedema (dropsy) in some cases. The post-mortem results of the infected fishes showed abdominal ascites (accumulation of fluid in peritoneal cavity), enlargement and congestion of the liver, kidneys, spleen and intestine with distension and congestion of the gall bladder.

The infected fishes revealed the presence of 468 bacterial isolates related to 8 bacterial genera and species, such as Aeromonas hydrophila (198), Pseudomonas fluorescens (102), Flavobacterium columnare (36), Klebsiella (48), Escherichia coli (24), Proteus (12), and Shigella (12). The pathogenicity of isolated strains revealed that Aeromonas hydrophila appeared to be highly virulent (100% mortality in infected groups) followed by Pseudomonas fluorescens (50%) and Flavobacterium columnare (37.5%).

Sensitivity test of isolated bacteria strains showed that Kanamycin and Nalidixic acid were the drugs of choice used for control and treatment of finrot disease. Kanamycin mixed with food has been reported as effective in curing finrot among ornamental fishes at 200–300 mg per 100 grams of food. It has also been reported that Kanamycin is absorbed from the water by fishes. Dose rate: 2–5 gm/L for 4–5 days. Afterwards make a 35–50% water change. Nalidixic acid (water soluble form) can be used as a oneto four-hour bath at a rate of 13 mg/L, repeat if needed. However, the use of antibiotics should be considered carefully before any application.

Bacterial finrot is very similar to fungal finrot with much the same symptoms. Fungal finrot looks like creamy or whitish, hairy-like growths. Bacterial finrot is whitish or gray, and although it causes the same amount of damage, the growths are not as prominent. Fungal finrot usually has a more uniform infection attacking all areas at the same rate. Bacterial finrot causes varying degrees of damage in some areas to little or no damage in other areas.

Bathing in a salt solution or various proprietary medications will usually prevent further development. There are several medications available commercially to treat finrot. One should follow the manufacturer's instructions for treatment, as different manufacturers use different chemicals and concentrations of the active ingredients. Malachite green at 1–3 mg/L of water for up to 1 hour has been effective in some cases.

Another effective treatment is immersion in a salt bath. Salt (Sodium chloride NaCl) is one of the most commonly used treatments for aquarium fish and is sometimes referred to as “aquarium aspirin”. Most tropical fish can tolerate a salt concentration of 1–3 g/L, and this level is not harmful to the biological filter. Short baths for 20–30 minutes at 10 g/L for fish species capable of tolerating certain salinity levels can be used. Most adult rainbowfishes seem to be able to tolerate salinities levels up to 17 g/L and juveniles 12 g/L. In a review conducted by Hart et al. (1989) the general conclusion was that most freshwater fish species in Australia appear quite tolerant of salinities up to 10 g/L.