We also have a disease section here on FishLore:White Spot (Ichthyophthirius)
A parasite that we will all encounter whilst keeping fish.
As an adult it is embedded in the skin of the fish causing irritation, your fish will probably be glancing off rocks and plants to alleviate the itching.
The parasite will feed and grow on the blood and skin cells of its host for a few days until it is fully grown.
It then bores its way out of the skin and drops off into the substrate, it then forms a cyst which then goes through rapid cell division until about 1000 young are released into the water to start the whole process again.
The whole process takes just five days at 27C.
There are cases where the parasite actually lies dormant in the skin of the fish and will not emerge until it is ready, making eradication quite difficult.
The best time to kill them is when they are free swimming and looking for a host.
Treatment must be external and aimed at the free swimming stage hence the need to dose again after a few days, suitable cures are widely available.