Bacterial Infections

• Goldfish are sometimes prone to skin ulcers. These commonly appear as pinkish white patches on the skin, sometimes featuring a white edge. Ulcers are usually caused by Aeromonas salmonicida, a type of bacteria. The illness can be progressive, with infection sites becoming ragged and raw-looking.
  
  More commonly called cotton mouth disease, columnaris is most frequently found around the mouth, resembling cottony patches, although it can also affect the skin. It is frequently misdiagnosed as a fungal infection but is actually caused by bacterium.
  
  

Parasites

• Also called white spot, Ich is named for the Ichthyophthirius multifiliis parasite, which causes the infestation. Ich presents as tiny white spots on the skin, fins and gills. Ich is triggered by poor water conditions. If left untreated, wounds from Ich can become infected by other pathogens.
  
  The parasite oodinium causes the disease Velvet. Velvet appears as a golden, velvety coating on the skin of the fish, which gives the illness its colloquial name: gold dust disease. This illness can be difficult to detect due to the gold color of the fish's scales.
  
  Implanting itself in the skin and gills of goldfish, the trichodina parasite causes intense itching and skin and gill irritation. Fish infected with the trichodina will commonly rub against rough surfaces. This can open wounds that may then acquire secondary infections. Poor water conditions are usually the cause of this parasite, occurring due to trichodina feeding on bacteria generated from unsanitary water.
  
  Costia is yet another parasite that infests goldfish. Infected fish will show patches of slime on the skin, which may become milky in color. This illness also affects the gills, and the fish may demonstrate difficulty breathing. If left untreated, costia is fatal.
  
  Fish lice, also known as Argulus, are tiny parasitic arthropods that are large enough to be seen with the naked eye. Fish lice move around freely on the skin of the fish; they feed on blood and body fluids. An infestation can be difficult to cure as fish lice are prolific and lay many eggs.
  
  

Fungal Infections

• Fungal infections often appear as fluffy, cottony growths on the skin. They are most often caused by the Saprolegnia or Achlya fungi. Fungal problems are rare in well-kept aquariums, although they may occur secondarily to other infections such as Ich.
  
  

Viruses

• Lymphocystis is a viral disease caused by the iridovirus that produces grayish-white lumps on the fins and skin. These lumps are caused by overgrown clusters of skin cells. Some fish that carry this disease do not exhibit symptoms. While Lymphocystis is viral, it can be triggered by stress from poor water conditions. Though unsightly, the disease is rarely fatal.
  
  

Flukes and Worms

• Body flukes, also referred to as shimmies, are parasites that resemble small worms. Fish that are infected can appear pale with a thicker-than-usual slime coat. Red spots or patches may appear on the skin, and the fish may rub against rough surfaces, similar to the behavior demonstrated in fish with trichodina infections.
  
  The anchor worm, scientific name Lernaea, is a type of parasitic worm that resembles a 1 cm-long filament of heavy thread protruding from the skin of the fish. Occasionally, the worm may exhibit two oblong egg sacs at the tip of its body.