White Spot or 'Ich' is a protozoan parasite that attaches itself to fish. It is present in water everywhere, and will be in almost all freshwater tanks. The parasite goes through several stages over about 10-15 days (temperature dependant), including a free-swimming stage that can rapidly infect other fish.
What it looks like:
The visible stage of Whitespot looks like grains of salt stuck to the outside of the fish. It can appear anywhere on the fish but often attaches to fins and gill plates.
What causes it:
White Spot is invariably caused by poor water conditions in combination with stress - it is a sign the the fish's immune system is failing, allowing the parasite to take hold. As certain stages of the parasite cannot live in water above 28 degrees, it should be less of a problem in a Discus tank as the water should be already be that warm.
What to do:
As it is often caused by poor water, test the aquarium water to determine if filters etc are operating correctly. Do a large waterchange. Raise the temperature to 30 degrees, but make sure that there is adequate water motion to maintain good oxygen levels in the water (as the temperature of water rises, it holds less and less oxygen) - often you will need to add an airstone.
There are several White Spot remedies available, most of which are organo-toxins that work by killing the free-swimming stage of the parasite. As Whitespot eggs can hatch out over a long period of time, most treatments take at least a week in order to break the lifecycle of the parasite - follow the instructions on the product and DO NOT OVERDOSE.