Shop safe
Shop safe

Basic Guides

Fish Diseases


In spite of all precautions, disease episodes may still occur in the aquarium. It is critical to monitor your fish regularly so that you notice any abnormal behaviour or differences in appearance which may signify the onset of disease. Prompt and correct diagnosis and treatment is the secret to increasing the chance of success when treating fish diseases.

Things to Look For :

Fish Appearance

Symptoms of Fish Disease
Definite Symptom Of Disease Possible Symptom Of Disease
Fish gasping. Rapid gill movement. Fish hanging near surface. Bacterial gill disease. Poisoning/Water quality
White Spot. Velvet. Slime Disease. Higher form parasite
Flicking & Rubbing White Spot. Velvet. Slime Disease. Higher form Parasite. Poisoning/Water Quality
Peppering of Gold Spots Velvet  
Patches of Slime Slime Disease. Poisoning/Water Quality  
White Spots (sugar grain) White Spot  
Disc or Wormlike attachments on body/gills Higher form Parasite  
Gills pale/eroded Higher form parasite. Bacterial gill disease White Spot. Velvet. Slime Disease
Cloudy Eyes Slime disease. Poisoning/Water quality  
Cotton wool Growths Mouth Rot. Fungus  
Fins eroded/opaque Fin Rot Internal Bacterial Infection
Mouth/Head Erosion Mouth Rot  
Eyes Swollen (Pop-eye)   Internal Bacterial Infection
Distended/Hollow Stomach   Internal Bacterial Infection
Holes/Ulcers   Internal Bacterial Infection
Fish Colour Darkens Poisoning/Water Quality Internal Bacterial Infection
Unexplained Deaths Internal Bacterial Infection  
Darting Around Poisoning/Water Quality  
Unable to maintain Balance Swim Bladder problems. Poisoning/Water Quality Internal Bacterial Infection

White Spot

White Spot is a protozoan infection caused in freshwater by Ichthyophthirius species and in salt water by Cryptocaryon species. These protozoan parasites spend part of their life cycle embedded in the skin of fish, feeding the tissue and body fluid. This on-fish stage is enclosed in a white cyst, which gives the classical (white spot) symptoms of this disease. The adult emerges from the cyst and drops to the floor of the aquarium where it forms into a protective capsule inside which it multiplies by division. Some days later the capsule bursts and releases hundreds of free-swimming infective parasites which search for a new fish host. Massive infestation, particularly of the gills, leads to respiration problems in the fish, salt balance difficulties, and eventually to death. The white spot life cycle duration depends on the temperature of the water, being only 10-12 hours at 26°C, but 3 weeks at 5°C. The white spot's free-swimming stage is the only stage of the life cycle affected by the treatment.

White Spot Treatment

A specific White Spot treatment (such as Interpet's Anti White Spot No.6) should be used. These products remain active in the aquarium for several days, so ensuring the free-swimming parasites are destroyed as they emerge from the capsules. You can assist rapid eradication of white spot by raising the aquarium temperature to 26°C, so shortening the life cycle.


This is caused in freshwater aquariums by protozoan parasite species called Oodinium and in marine water by Amyloodinium. These protozoan parasites have a very similar life cycle to white spot, however, the cysts are much smaller, being about 1 mm in diameter, and far more numerous, giving the appearance of a peppering of gold spots, or a velvet like texture to the fish's skin.

Slime Disease

This disease is also caused by protozoa e.g. Chilodonella, Ichthyobodo (Costia), Trichodina and Brooklynella species, and Trematodes (flukes) e.g. Gyrodactylus and Dactylogyrus species.

These parasites spend the whole of their life cycle on the fish, although infective young parasites may be released into the water to spread to new hosts. The parasites live on the skin and gill surface, eating gill and skin tissue debris. If their numbers increase rapidly, they may irritate the skin and gills, causing excessive mucus production and usually killing the fish by smothering the gills and hence suffocation.

Slime Disease Treatment

The correct treatment (such as Interpet's Anti Slime and Velvet No.7) quickly and effectively eradicates gill and skin parasites and removes the mucus build up, therefore aiding the fish's respiration.

Higher Parasite Forms

These come in the form of crustaceans, e.g. anchor worm (Lernaea species) fish lice (Argulus species) gill maggots (Ergasilus species) and fish leeches. These higher forms of parasites are fortunately not common in aquarium fish.
Treatment for Higher Form Parasites Treatment should be with a product such as Interpet's Anti Crustacean Parasite No. 12. This powerful treatment eliminates these difficult to eradicate parasites, but should be used with care!!


Finrot is caused by Aeromonas and Pseudomonas species of bacteria found in all aquariums. The fin and ray tissue becomes opaque, blood streaked and eaten away. It is important to catch this erosion before it reaches the fin base, as this leads to the fish's death. Susceptibility to this disease is commonly caused by poor water conditions or damage resulting from fin-nipping fish. It is therefore necessary to deal with the cause of the problem to prevent a recurrence.

Finrot Treatment

Treatments (such as Interpet's Anti Fungus and Bacteria No.8) destroys the bacteria, halting fin erosion. It may take some time for the fin to grow back. The medicine remains active for several days thus preventing a rapid recurrence of this disease.

Mouthrot / Mouth Fungus

This is caused by a bacterium, Flexibacter Columnaris. The symptoms of this disease are typically erosion of the mouth, commonly associated with a fungal like growth, which is in fact colonies of the column-like bacteria. Like finrot, mouthrot is associated with poor water conditions which must be corrected.

Treatment for Mouthrot / Mouth Fungus

Treatments (such as Interpet's Anti Fungus and Bacteria No.8) destroys the bacteria, halting mouth erosion although it may take some time for regeneration. The medicine remains active for several days thus preventing an immediate recurrence.


Fish fungus is caused by fungus species such as Saprolegnia. Fungus spores are present in all freshwater aquaria. A disease outbreak is usually a secondary infection of an area of the fish's skin or gill barrier which has been damaged. A Saprolegnia growth looks like cotton wool. It is essential to prevent the fungus spreading deep into the tissue or over the gills. Livebearers are particularly susceptible to fungal problems which can usually be prevented by adding 0.1% (1 gm per litre) of aquarium salt to the tank.

Fungus Treatment

Treatments (such as Interpet's Anti Fungus and Bacteria No.8) stops the fungus spreading and destroys it. The medicine remains active for several days to ensure full healing occurs.

Internal Bacteria

These diseases are caused by a variety of bacteria species, e.g. Pseudomonas flourescens, Aeromonas hydrophila and Virio marinium. Internal bacterial infections spread throughout the fish's tissue and internal organs. Symptoms vary, depending on the acuteness of the disease. In some cases the fish will just darken in colour, become listless, stop feeding and die. In others severe symptoms exist, ulcers, abdominal and eye swelling, destruction of the central nervous system, and then the fish dies. It is essential to catch these diseases as early as possible or treat as a precaution as soon as they are suspected.

Treatment for Internal Bacteria

Previously, the only effective treatment against internal bacteria was veterinary prescribed antibiotics. New medicines (such as Interpet's Anti Internal Bacteria No.9) are now available over the counter. These easy treatment methods improve the chances of successful treatment of internal bacterial infections. Bacterial problems such as dropsy, ulcers and septicaemia are very difficult to cure if the disease is too far advanced, but the medicines will help to prevent further deaths.

Bacterial Gill Disease

Bacterial gill disease is caused by bacteria (e.g.Flexibacter species). Bacterial erosion of the delicate gill membrane causes suffocation of the fish. Bacterial gill disease often occurs after the gills have been damaged by poor water quality. Affected gills are pale or grey/brown in colour with heavy mucus production and clear signs of erosion.

Treatment of Bacterial Gill Disease

Treatment with medicines (such as Interpet's Anti Internal Bacteria No.9) plus the addition of 0.2% aquarium salt is recommended. Water conditions must be checked, as poor water contributes greatly to the onset of this disease.

Swim Bladder Problems

Swim bladder problems are caused by bacterial and viral infections, or hereditary problems. The swim bladder helps the fish to control its buoyancy, allowing it to maintain its position in the water. Internal bacterial infections can affect its function, making it difficult for fish to maintain position. Fancy goldfish with their short body form are particularly prone to this problem.

Treatment for Swim Bladder Problems

Raising the temperature to 27°C (80°F), adding aquarium salt to a level of 0.5% (5 gm per litre), and using a treatment such as Interpet's Swim Bladder Treatment No.13 is recommended. You should also feed sparingly.

Poisoning/Water Quality Problems

There are several causes for this type of health problems:

  • Poor water quality, i.e., high or low pH, high ammonia/nitrite levels.
  • Untreated tap water containing chlorine or heavy metals (copper)
  • Medications. There may have been an overdose, or a particular species could be susceptible to an active ingredient.
  • Household toxins e.g. paint fumes, polish etc.

Treatment for Poisoning/Poor Water Quality Test for ph, ammonia and nitrite using a test kit. Use carbon in the filter system or in a separate box filter in the corner of the aquarium. Carry out several large scale water changes. Ensure the tap water is treated with a conditioner to remove chlorine and heavy metals. Identify the source of the problem and eliminate it.


  • Always follow the dosing instructions on the bottle.
  • Remove any carbon from the filters before using any disease treatments.
  • Do not use more than one disease treatment at a time.
  • Keep all medicines away from children.
  • If in doubt seek advice from your aquatic retailer.

Interpet, the specialists in aquarium and pond care, have been developing and manufacturing aquarium products since 1952. Interpet's product range has been designed to achieve the goal of trouble free fishkeeping. We are indebted to them for permission to use the information on this page.