What Is White Spots?
This is one of the most common disease that affects aquarium fish. This disease is so common because part of the lifecycle of white spots is in a egg-like shell (called a "cyst"). Without medication, this "egg" is very hardy while it floats around inside your aquarium or on the tank floor. If you have several fish tanks, these 'eggs' can be transferred from one tank to another when you use the same net to catch fish, or hitchhike when you transfer fish/plants/decorations around. In fact, the most common way you get them into your aquarium environment is when you buy fish from the fish shop!
When the time is ripe, these 'eggs' burst open and spreads hundreds of spores into the water. These spores then 'fly around' and attach themselves to your fish. Once on the fish, they dig below the surface of the skin and then eat the fish tissue to grow. Once adult, they drop off the fish like the 'eggs' that they came from, and then burst with hundreds of spores. This cycles continues until they are killed. If left alone, they can multiply more and more and infect all the fish they have access to.
Attacking White Spots
White spots are very tough to kill when they are hiding below the fish's skin, and they are also protected when they are in the 'egg'. The window of opportunity to attack them where they are weakest is when they are in the spores form, after they just burst from the 'egg'.
Most white spots cases happen when the temperature is cooled. To treat white spots, place a heater in your aquarium and slowly increase the temperature slightly. Adding salt will help too. Since temperature and salt tolerance is dependent on your fish species, I try not to give any specific number. Increase aeration if you can, because a higher temperature reduces the amount of dissolved oxygen. Also, the fish gills may be attacked by the white spots and give your gish more difficultly breathing.
Besides increasing temperature and salt, you can also get commercial medication from fish stores. All of them should work, however, they may affect your biological filter so keep a close check on your ammonia and nitrites. If you use commercial medication, you should remove any activated carbon if you are using them, because they can adsorb the medication or make it less effective.
Disadvantages of chemicals that treats white
Copper - toxic to freshwater fishes/shrimps
Methylene Blue - harmful to aquarium plants
Formalin - kills beneficial bacteria from filter
Malachite Green - carcinogen (cause cancer!)