White spot disease is caused by a parasite called Ichthyophthirius multifilis. This disease is also called Ick or occasionally Ich or Ichy https://expressfishings.com/.
The fish has white spots on its skin. The spots are about the size of a pinhead and the fish can look as if it has been sprinkled with salt or sugar grains. The parasite also attacks the gills of fish. This is more difficult to see. The gills may look more red than usual, but this is hard to see, and excessively red gills can be caused by a number of things. The gill infection makes it more difficult for the fish to absorb Oxygen from the water and infected fish can show signs of being short of Oxygen like “gasping” at the surface, or apparently breathing very fast. This shortage of Oxygen can be caused by many things.
Sometimes fish will swim down and try to rub their skin against objects. This is called “flashing” and can be caused by any skin irritation.
Sometimes fish show no obvious symptoms, but simply die. If a fish dies you should take a very close look at all the fish in the tank.
This is a very common disease of fish. The parasite is present at low levels in most aquariums, often without causing any trouble. Most fish have been exposed to this parasite and have developed some immunity. Those fish that have been raised in the complete absence of the parasite will not have this acquired immunity and will be very vulnerable to infection.
The statement that this parasite is present in most aquariums is often misunderstood. Ichthyophthirius Multifilis cannot lie dormant for long periods. It survives by living on fish. An aquarium might be empty of fish for a month. It would be free of the white spot parasite. Then a fish was bought which was free of any visible disease and then quarantined. This fish could be introduced into the empty tank and develop white spot. The erroneous conclusion might be drawn that either the empty tank had dormant white spot, or that the quarantine was not correctly done.
What would actually have happened would simply be that the fish had a white spot infection without any symptoms. A successful parasite does not make its host ill. If the parasite wiped out all the fish in the aquarium, pond or lake it was in, the parasite itself would also die. In the wild the white spot parasite is apparently successful and most of the time does not kill its host. In the unnatural ecosystem of an aquarium it can easily get out of balance and kill all the fish. This is not only fatal to the fish; it is also fatal to the parasite.
The ideal parasite is one that actually gives some advantage to its host. As far as I am aware, having the white spot parasite is no advantage to fish, but other parasite/host relationships may have developed into symbiotic ones where both organisms get an advantage.
If something stresses the fish, their immune system often becomes less effective. The same effect can be observed with people. You are much more likely to get both minor and major diseases when you are under stress.
There are many things that can stress fish. One very common one is simply being caught, put into a plastic bag and transported to a new home. A common time for an outbreak of White Spot is just after a new fish has been added. Some people incorrectly assume that the new fish has introduced the parasite. They may then go back to the shop they brought it from and see that the tank the fish came from is perfectly all right.
Other types of stress include changes in temperature, pH, dH or any other water parameter.
Ichthyophthirius Multifilisis an obligate parasite. This means that it can only live in the presence of fish. The actual visible white spots are the feeding stage, called a trophont. The trophont grows and then drops off the fish, falling to the bottom of the tank and forms a cyst called a tomont. Inside the tomont as many as 1000 tomites can form. The tomont opens and the tomites go into the water.
The time it takes for Ichthyophthirius Multifilis to complete its life cycle depends on the temperature of the water. At 6 degrees C (43 degrees F) is gets through its life cycle in about 55 days, while at 29 degrees C (84 degrees F) it completes its cycle in only about 4 days.
The tomites have to find a fish quickly or they will die. At normal tropical fish tank temperatures they only have about 2 days to find a fish to infect.