Caused by Ichthyopthirius multifiliis. The white spots on the skin, gills and fins are individual protozoan cells that are under the skin and feed on the body fluids and cells. They then punch out of the skin and fall to the bottom of the pond, collect together and begin breeding, the offspring then re-invest the fish.
As well as white spots symptoms are scratching and swimming into the water inlet, failure to feed and lethargy. It is fatal if untreated. Fortunately commercial white spot remedies are widely available.
Ich is caused by a ciliated protozoan of the genus Ichthyophthirius – Ich is simply an abbreviation of the word Ichthyophthirius. Some people prefer to spell it Ick, perhaps because they find the disease icky? Ich is also known as White Spot Disease since it causes small cysts to form on the skin of infected fish. If you look at the protozoan under a microscope, you will see a round rolling mass with a u-shaped nucleus. This parasite is completely surrounded by cilia.
The Ich parasite starts its life as a free swimming organism that moves around in the aquarium, looking for a suitable host. When it finds a fish, it will attach itself to the outer layer of the fish's skin and start feeing on its bodily fluids. It can also attach itself to the gills, thus making it hard for the fish to breathe. Soon, the parasite will have developed a protective outer shell, a so called cyst, and it is this shell that we can see as a small white grain on the skin of infected fish. A fish heavily infested with Ich parasites can look as if someone had sprinkled it with grains.